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Pre- and postcapillary resistances in cat mesentery
MICROVASCULAR
RESEARCH,
7,351-361 (1974)
Pre- and Postcapillary Resistances in Cat Mesentery1s2 KITTY FRONEK AND BENJAMIN
W. ZWEIFACH
AMES-Bioengineering, University of California at San Diego, Lu JoNa, California 92037 Received March 6, 1973 The hemodynamic behavior of the isolated autoperfused intestinal loop (including mesenteric segment) of the cat was studied in terms of inflow and outflow pressures (BP and VP), arteriolar and venular pressures (P,, and P,), and blood flow (Q). The regional vascular resistance [RR = (BP - VP)/Q] was subdivided into a precapillary resistance [P,R= (BP - P.)/QJ, capillary resistance [CR= (P. - P,)/Q], and postcapillary resistance [P,R = (PV- VP)/Q]. With normal perfusion pressure it was found that: P,R = 68%, P,R = 20.5 %and CR=ll.S %.Twodifferentresponses were noted when perfusion pressure was lowered to 20mm Hg. In the first group, precapillary resistance fell significantly, whereas postcapillary resistance did not change. In the second group, the regional resistance increased due to the increase in precapillary resistance and a significantly greater increase in postcapillary resistance. The regulatory mechanism in the first group is based on local readjustment of the precapillary vasculature with no participation of postcapillary vessels. The second group of responses appears to be related to completely dilated vessels, where regulation is achieved by increasing postcapillary resistance in an attempt to maintain the capillary pressure and homeostatic balance.
INTRODUCTION Local adjustment in the terminal vascular bed is one of the principal mechanisms which determines fluid and nutrient exchange. To date the basic information on these regulatory mechanismshas beenalmost exclusively derived from indirect measurements. Folkow and co-workers (Folkow, Lundgren, and Wallentin, 1963)analyzedtherelationship between flow resistanceand regional blood volume in the intestine of the cat using the “isovolumetric” preparation. Johnson and Hanson (Johnson and Hanson, 1962) and subsequently Johnson (Johnson, 1960) utilized a combination of the isogravimetric method and direct cannulation of small arteries and veins in the isolated intestinal loop to calculate the pre- and postcapillary resistances. The autoregulatory response to an increase in perfusion pressure on the arteriolar diameter in the mesoappendix of the rat was documented by Baez (1968). Similarly, Johnson (Johnson, 1968)investigated arteriolar diameter changesin the cat mesentery of an isolated autoperfused intestinal loop with stepwise reduction of the perfusion pressure. In these studies, it was found that when perfusion pressureis reduced, there is a decreasein the pre- to postcapillary resistance ratio, which was interpreted to be a 1 This work was supported by a NIH Grant No. USPHS HL-10881. z Part of this work was presented at the 20th Annual Meeting of the Microcirculatory Atlantic City, New Jersey, April 9, 1972. 351 Copyright 0 1974 by Academic Press, Inc. All rights of reproduction Printed in Great Britain
in any form
reserved.
Society in
352
FRONEK AND ZWEIFACH
consequence of autoregulatory responses. Additional evidence suggested that this local regulatory processtakes place in vesselssmaller than 0.1 mm, and most likely in arterioles smaller than 50pm. In recent studies, Richardson and Zweifach (Richardson and Zweifach, 1970; Zweifach and Richardson, 1970) reported the micropressure results from arterioles and venules utilizing Wiederhielm’s (Wiederhielm et al., 1964) electronic servonulling pressure measurementmethod in the cat mesentery. This method was utilized to determine: (a) the pressure distribution in the microvessels as related to their size; (b) the pre- and postcapillary resistanceswith referenceto the size of the microvessels; and (c) the respectiveparticipation of vesselssmaller than 25 pm in the autoregulatory response. METHOD Experiments were performed in the mesentery attached to the isolated loop of 10 cats weighing on the average 2.5 kg under pentobarbital anesthesia (30 mg/kg). The isolated autoperfused intestinal loop was prepared according to the method described by Johnson (Johnson, 1960; Johnson and Hanson, 1962)and Selkurt et al. (Selkurt et al., 1958).A segmentof the terminal ileum, 25-50 g in weight, was exteriorized through a midline incision and isolated from the remainder of the intestine by severing all mesenteric connections, leaving only the arterial and venous lines intact. Denervation resulted during the process of dissecting and cannulating the artery and vein. After heparinization (1000 units/kg), the mesenteric artery was connected to the femoral supply artery via polyethylene tubing. A 1 in. piece of silastic tubing was inserted into the arterial inflow line. A fine screw clamp placed on the soft silastic tubing permitted reduction of the inflow pressure head in fine steps.In each experiment, the initial pressure was reduced to 100 mmHg, subsequently reduced by 20 mmHg steps until 20 mmHg was reached. The pressure changes were determined 60-90 set after each step of blood pressure reduction. The inflow arterial blood pressure was measureddistally from the clamp by a Statham pressure transducer P23Db. The terminal portion of the mesenteric vein was cannulated, and blood was returned via the femoral vein after passing through a drop counter and a peristaltic pump triggered by a photoelectric sensor. The right atrium could not be used as the phlebostatic level for the outflow pressure because the mesenteric veins are drained by the portal vein in which the pressure, under physiological conditions, varies from 12 to 16 cm H,O. Therefore, the venous pressure was adjusted to a level where additional lowering of the outflow reservoir tubing was not followed by a further increase in blood flow. This venous outflow pressure was assumedto be the control pressure throughout the experiment. It varied from animal to animal between47 cm HzO. This pressurewas measured by a Statham pressuretransducer P23BB, connected by a T-junction to the outflow line, and corrected by the pressuredrop due to the resistanceof the outflow tubing. The perfused segment of intestine and mesentery was placed on a heated platform in an intravital microscope and continuously irrigated with a solution containing 8.7 g/liter NaCl, 0.3 g/liter KCl, 0.3 g/liter CaC&, and 10 g/liter gelatin buffered with Na,CO,, and maintained at body temperature. Glass micropipettes with sharpened tips 3-5 pm were filled with 2 M NaCl and were inserted into selectedmicrovesselswith the help of a micromanipulator. Blood pressure
PRE- AND POSTCAPILLARY
RESISTANCES
353
in the vesselsof the mesentery was measured directly with the servonulling method developed by Wiederhielm et al. (1964) and modified by Intaglietta, Pawula, and Tompkins (1970). In these preparations, it was possible to measure simultaneously inflow and outflow pressures, arteriolar and venular pressures,and total blood flow. This data was utilized to calculate pre- and postcapillary resistancesand their ratio. The same flow was used to calcu!ate resistancesin the various segmentssince the same amount of blood must flow in and out of the preparation under steady state conditions. Johnson (Johnson, 1960; 1968)has shown that under conditions of stepwise reduction of inflow pressure, autoregulatory responseswere equally exhibited by the mesentery and the whole intestine tested with the isogravimetric technique as well as with the measurementsof arteriolar diameter changesin the mesenteryonly (Johnson, 1968). Therefore, it can be assumedthat flow changesin the mesentery reflect the corresponding changes in the intestinal wall. As a consequence,if regional resistance is taken to be the sum of resistances,then P,R + P,R + CR = RR. The resistances(Fig. 3) for any particular segment of mesenteric vasculature were calculated as follows: RR = (BP - VP)lQ,
(1)
where RR regional resistance BP inflow arterial blood pressure VP outflow venous blood pressure
Q outflow for the whole preparation P,R = (BP - P,)/Q
(2)
P,R = (P” - VP)/Q
(3)
CR = (Pa - PJQ,
(4)
where P,R precapillary resistance Pa blood pressure in given feeding arteriole P,R postcapillary resistance P, blood pressure in draining venule CR capillary resistance as calculated here represents the lumped resistancesfor
the vascular network of vesselsin the range of 6-20 pm on the arteriolar and venular side. Changes in resistance due to the BP manipulation were compared with respective control values and analyzed by Student’s t test and the limit for statistical significance was considered P co.05 or less. Diameters of all vesselsand their position in the terminal vascular bed were determined from photographs at microscopic magnification of 200-300x. Illumination was provided by a 100 W Xenon arc lamp. RESULTS The anatomy of selectedmicrovesselswhere the measurementshave been performed were either (a) continuously aligned paired arterioles and venules (Fig. 1) which are usually defined as arcuate vessels,although the artery and vein do not always servethe
354
FIG. 1. Microphotograph pipettes.
FRONEK AND ZWEIFACH
of paired arteriole and venule. Arrows indicate the position of micro-
samearea (Fraser and Wayland, 1972),or (b) vesselsin which one feeding arteriole and venule served to supply and to drain a well-defined region (Fig. 2). A total of 104 vesselsstudied represent the statistical population of the respective sites. The distribution of blood pressures and vascular resistances in arterioles and venules.
In view of differencesin systemic blood pressurein individual cats, the mean pressures in microvesseIsare expressedas percentageof the mean systemicblood pressure(Table I). Blood pressure in the small arterioles of approximately 20 pm in diameter is of the order of about 38 % of the mean arterial blood pressure, while in venules of the same diameter the pressure is approximately 25%. The pressure drop across true capillaries (exchange vessels) in the mesentery averaged 127; of the systemic mean arterial blood pressure or 14 cm H20. Venular pressuresin the mesentery were higher than venular pressuresin other vascular regions such as skin or skeletal muscle, because under normal conditions the splanchnic venous (portal) pressure is the highest in the organism. This is well documented by the results obtained in our laboratory from measurementsperformed on cat mesentery in situ (Zweifach, 1933).
D* (wn)
6.9 3
0 3
.-
11.5 3
5
6.52
5
5.5 5
3.35
4.42 25
1.82 25
126.7 6.2 26
46.2 3.29 17
14.63 1.23 17
Diameter of microvessels < 20 pm
4.1 38
38 2.32 17
2.06 25
Diameter of microvessels 50-21 ,um 53.7 132 67.0 37.6
23
6
Diameter of microvessels 100-51 pm 56.2 133 14.5 65
11.4 3
__32 2.16 14
29.0 2.3 22
18
2.13
26.8
751.1 1.13 14
36.4 2.16 22
18
2.7
65
(CAT MESENTERY)
26.4 2.47 14
25.0 1.42 22
18
1.21
19.8
--16.1 1.33 26
16.0 0.77 38
23
1.1
17.6
14.8 3.5 3
5.43 0.40 26
5.4 0.54 38
3.68 0.29 17
2.59 0.35 25
5
0.26
0.20 23
2.26
0.59 0.24 3
5.25
5.51 0.84 3
0.63 0.21 14
1.56 0.34 12
2
0.18
2.03
1.13 0.10 14
1.09 0.05 22
18
0.09
0.94
a Abbreviations: BP, mean arterial blood pressure; Pa, mean arteriolar pressure; P,, mean venular pressure; D., arteriolar diameter; D,, venulardiameter; SE, Standard error of mean; N, number of measurement. Resistances are expressed as mmHg/ml-i/min-l/lOO-l g of tissue. For details see text.
Number
SE
Average
Number
SE
Average
Number ~__
SE
Average
Average SE Number
p. (cm HA2
Diameter of microvessels < 100 ,~m 90 118 107 200
BP (cm HA3
1
CONTROLHEMODYNAMICDATA OF THE TERMINAL VASCULAR BED”
TABLE
-
3 2 3 E
B
1
2
$
3 3
5
P
356
FRONEK AND ZWEIFACH
FIG. 2. Microphotograph of an area with one feeding arteriole and a draining venule. Arrows indicate position of micropipettes.
Sixty-eight percent of RR is contributed by the precapillary vessels,i.e., vesselswith a diameter greater than 20 pm but still accessibleto intubation by the micropipette without interrupting flow (Fig. 3). There is no significant pressure difference between the group of arterioles from 100 to 50 pm, and the group from 50 to 21 pm, while the average pressure in the <20pm vesselsis significantly smaller. This finding suggeststhat the 50 to 21 pm vessel group is the primary determinant of P,R. 20.5 % of RR is generated by vesselsof the samesize, but on the venular side. The remainder constitutes the CR. This portion of vascular resistance is of composite origin and, due to the technique 60 40
ml4
20 i O
820 -A
iGT BP-‘Pa Q
I%+
21-50
51 - 100,(c,ons
P+ I
RR-Bp-vp
Q FIG. 3. Resistances distribution related to the size of the vessels. Resistances are expressed as per-
centage of total RR. Dotted columns summarize data from arterioles-plain columns from venules. Number of vesselsin each size range is the same as in Table I. For details see text.
PRE- AND POSTCAPILLARY
RESISTANCES
357
used (size of the micropipette), someparticipation of precapillary sphincters cannot be excluded. Resistance changes due to decrease ofperfusion pressure. Control data were determined in 22 paired vessels,feeding and draining the sameregion. Subsequently,BP wasreduced in a stepwisefashion as described in the methodical part. BP was reduced in four steps and repeated usually in two to four runs. A new run was initiated only after control pressure conditions were reestablished. The arteriolar and venular resistanceswere routinely calculated from measurementsin vesselswith a diameter of 25-15 pm. The changesin resistanceare summarized on Figs. 4 and 5. Results from six animals can be placed in a group (A, Fig. 4) characterized by the fall of RR with each decreaseof perfusion pressure, mainly asa consequenceof a decreaseP,R. When the perfusion pressure
20
40
INFLOW
60
a0
100
p mmHg
FIG. 4. Resistance changes in Group A due to decrease of the perfusion pressure. Resistances are expressed as cm H20/ml/min/100 g. RR, regional resistance; P,R, precapillary resistance, P,R, postcapillary resistance. (For details see text.)
was lowered from 100to 20 mmHg, RR fell from 7.2 f 0.62 (SE) cmH,O/ml/min/l00 g to 4.4 f 0.57 (SE). At the sametime P,R fell from 5 f 0.56 (SE) to 2.15 10.3 (SE) cm H,O/ml/min/lOO g. P,R changed very little, but showed a tendency to increase from 1.35 + 0.15 (SE)cm H,O/ml/min/lOO g to 2.16 + 0.5 (SE). This changeisnotstatistically significant. In the remainder of four animals-Group B, Fig. 5-RR increased as the perfusion pressure was lowered. This rise in resistancewas associatedwith a substantial increase in P,R’s. When BP was lowered progressively from 100 to 20 mmHg, RR rose from 7.17 f 0.4 (SE) cm H,O/ml/min/lOO g to 13.4 f 0.9 (SE). The increase in P,R from 4.4 f 0.15 (SE) cm H,O/ml/min/lOO g to 6.14 + 0.74 (SE) was significant (t = 6.776) from 1.46 f 0.18 (SE) to 4.83 f 0.46 (SE) cm H,O/ml/min/lOO g. If the sameresults are expressedas pre- to postcapillary resistanceratio, a decreasecan be observedfrom 3.78 to 1.34 in Group A and a similar drop from 3.23 to 1.29 in Group B. Even though the pre- to postcapillary resistance ratios in both groups are similar, the mechanism of
358
FRONEK AND ZWEIFACH
their changein the two groups is reversed: in A, the pre- to postcapillary resistanceratio decreasesdue to a decreasein P,R, and in group B, due to a predominant increase in
P,R. 14
10
a a
6
a
4
a”
2
+kT%++? INFLOW
100
P mmklg
FIG. 5. Resistance changes in Group B due to decrease of the perfusion pressure. Description same as in Fig. 4.
C B y=xo544+J.i47
0
4
8
12
I6
20
Pv cmHLO
24
28
32
36
0
4
0
12
I6
20
24
2.3
32
36
Pv cmHrO
FIG. 6. Flow to venular pressure relationship due to decrease of perfusion pressure. Flows are expressed in ml/min/lOCl g of tissue, venular pressures in cm H,O. Right side summarizes data for Group A, left for Group B.
PRE- AND POSTCAPILLARY
RESISTANCES
359
The changesin postcapillary vesselsduring reduction of BP were analyzed in terms of pressure flow relationship. From the comparison of the regression lines and their intercepts for both groups (Fig. 6), it can be seenthat in Group B the venular pressures are higher than in Group A for the sameflow rates. No significant difference was found between both regression lines. Vesselsselected for this correlation were of the same diameter. DISCUSSION Direct pressure measurementsobtained in conjunction with intravital microscopy, and the subsequent calculation of the respective resistances,provides a better understanding of the hemodynamics of the terminal vasculature. The first significant pressure drop under steady-rate conditions occurs on the arteriolar side in the group of vessels from 100 to 50 pm diam. A second large pressure drop is in vesselswhich range from 20 pm diam to true capillaries. This pressurerelationship is different from the one found in skeletal muscle (Fronek and Zweifach, 1973),where the largest pressuredrop occurs in arterioles between 35 and 10 pm diam. The averagevenular pressurein the smallest veins is 24 mmHg which is the nearest value to the capillary pressure. In comparison with the results reported by Richardson and Zweifach (1970) in the sameexperimental arrangement, the present average value is 4 mmHg lower. Johnson, by using the isogravimetric method in the autoperfused isolated intestinal loop (Johnson and Hanson, 1962), obtained even lower values, and questioned the use of the isogravimetric technique in this type of preparation. It should be noted, however, that he assumed, on the basis of plasma protein content, an effective capillary pressure of 20-21 mmHg (Johnson and Hanson, 1962). The presented venular pressuresare in agreementwith in situ data from the cat mesentery(Zweifach, 1973). In the present experiments, the averagepre- to postcapillary resistanceratio of 3.5 is smaller than that obtained for the hind limb by Pappenheimer and Soto-Rivera (Pappenheimer and Soto-Rivera, 1948)and by Mellander (Mellander, 1960).In comparing our data and previous estimates, several factors should be considered. Pappenheimer and Soto-Rivera (Pappenheimer and Soto-Rivera, 1948) derived the RR ratio from measurements on the hind limb, while our results were obtained from the isolated intestinal loop. As has been pointed out, the venous pressuresin the splanchnic region are higher than in the skeletal muscle. Secondly, the pre- and postcapillary resistances were calculated in Pappenheimer and Soto-Rivera’s experiments on the basis of mean capillary pressure measurements,whereas our pressure data were obtained from preand postcapillary vesselsdirectly, so that the resistance values are necessarily slightly lower. For the same reason, the resistance between the arteriolar and venular site of measurements--capillary reistance-is slightly larger. When the perfusion pressure is lowered from 100 to 20 mmHg, the pre- and postcapillary ratio drops almost to 1, indicating that the pre- and postcapillary resistances became equal, while CR does not change. As has been shown, such a change in the resistanceratio either can be due to a decreaseof P,R (as found in Group A), or due to a predominant increase of P,R (Group B). The regulatory mechanism responsible for the type of adjustment characteristic of Group A is similar to that reported by Johnson (Johnson, 1960),and has been explained on the basis of the metabolic or the myogenic
360
FRONEK AND ZWEIFACH
theory, or both (Johnson, 1960; Johnson and Hanson, 1962). In such animals, local regulation could be achieved by readjustments of the precapillary vasculature with essentially no participation of the venular segment. The reaction pattern in this group does not support Johnson’s findings on venoconstriction. It should be pointed out that Johnson’s results are basedon the cannulation of small veins while this report describes measurementsin veins of 25 ,nmdiam, not accessibleby cannulation. It is possible that the effect of distally located venoconstriction could be mitigated to some extent by a sufficiently compliant capacitance system located between the site of constriction and the site of micropuncture of the venule. On the other hand in Group B, the response to decreasing perfusion pressure was characterized by a substantially greater increase in venular resistance.The explanation for this type of response is not clear. It is possible that in these casesthe precapillary resistancevesselscan no longer undergo physiologic dilatation, so that an increase in P,R, with a reduction in BP, balances capillary pressure and fluid movement equilibrium. Support of this hypothesis is provided by the observation that while the RR is the same in both groups, the P,R in Group B at the beginning of the run was significantly lower (P ~0.05). Furthermore, in these cases,reactive hyperemia was substantially reduced after the release of occlusion (43 vs 93%). There is no simple explanation to the increasing P,R in the nonautoregulating group. It is possible that thin-walled vesselsunder low tension, passively decreasein diameter, when perfusion pressure is lowered (Nagle et al., 1968).This factor does not seemto be decisiveunder thesecircumstancessincethe venular pressurescorresponding to similar flows are higher in the nonautoregulating group (Fig. 6). If the increase in P,R were a function of simple recoil effect, the pressuresin Group B would be lower or at least equal to pressuresmeasuredat corresponding flow rates in the autoregulating group. The participation of extrinsic nervous influence can be ruled out in view of the surgical isolation of the mesentery. On the other hand, it is possible that in nonautoregulating preparations, the arteriovenous reflex is quantitatively more significant (Hanson and Johnson, 1962). The interplay between pre- and postcapillary resistancesrepresentsan active physiological mechanism which is instrumental in maintaining capillary pressure and fluid equilibrium. Summarizing the responsesto stepwise reduction of perfusion pressure, it seemsthat this can be achieved either by decreasing the precapillary resistance or increasing the postcapillary resistance-in both cases decreasing the pre- to postcapillary resistance ratio. REFERENCES 1.
S. (1968). Bayliss response in the microcirculation. Fed. Proc. Fed. Amer. Sot. Exp. Biol. 27, 141%1415. 2. FOLKOW,E., LUNDGREN,O., AND WALLENTIN,I. (1963). Studies on the relationship between flow resistance, capillary filtration coefficient and regional blood volume in the intestine of the cat. Acta &and. Physiol. 57, 270-283. 3. FRASHER,W. G., JR., AND WAYLAND, H. (1972). A repeating modular organization of the microcirculation of cat mesentery. Microuasc. Res. 4, 62-76. 4. FRONEK, K., AND ZWEIFACH, B. W. (1973). Comparison of the effect of Papaverine on terminal vasculature of the cat mesentery and skeletal muscle. Presented at the 21st Annual Meeting of the Microcirculatory Society, Atlantic City, April 14, 1973. BAEZ,
PRE- AND POSTCAPILLARY
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5. INTAGLIETIA, M., PAWULA, R. F., AND TOMPKINS,W. R. (1970). Pressure measurements in the mammalian microvasculature. Microvasc. Res. 2,212-220. 6. HANSON,K. M., AND JOHNSON,P. C. (1962). Evidence for local arterio-venous reflex in intestine. J. Appl. Physiol. 17, 509-513. 7. JOHNSON,P. C. (1960). Autoregulation of intestinal blood flow. Amer. J. Physiol. 199,311-318. 8. JOHNSON,P. C., AND HANSON,K. M. (1962). Effect of arterial pressure on arterial and venous resistance of intestine. J. Appl. Physiol. 17,503-508. 9. JOHNSON,P. C. (1968). Autoregulatory response of cat mesenteric arterioles measured in vivo. Circ. Res. 22,199-212. 10. MELLANDER,J. (1960). Comparative studies on the adrenergic neurohormonal control of resistance and capacitance blood vessels in the cat. Acta &and. Physiol. 50,176-186. 11. NAGLE, F. J., SCOTT,J. B., SWINDALE,B. T., AND HADDY, F. J. (1968). Venous resistances in skeletal muscle and skin during local blood flow regulation. Amer. J. Physioi. 214,885-891. 12. PAPPENHEIMER, J. R., AND SOTO-RIVERA,A. (1948). Effective osmotic pressure of the plasma protein and their quantities associated with the capillary circulation in the hind limb of cats and dogs. Amer. J. Physiol. 152,471491. 13. RICHARDSON,D. R., AND ZWEIFACH,B. W. (1970). Pressure relationship in the macro and microcirculation of the mesentety. Microvasc. Res. 2,474-487. 14. SELKURT,E. E., SCIBETTA,M. P., AND CULL, B. E. (1958). Hemodynamics of intestinal circulation. Circ. Res. 6,92-99. 15. WIEDERHIELM,C. A., WOODBURY,J. W., KIRK, S., AND RUSHMER,R. F. L. (1964). Pulsatile pressure in microcirculation of the frog’s mesentery. Amer. J. Physiol. 207,173-176. 16. ZWEIFACH,B. W., AND RICHARDSON, D. R. (1970). Microcirculatory adjustments of pressure in the mesentery. 6th Eur. Co& Microcirculation, Aalborg, 248-253. 17. ZWEIFACH,B. W. (1933). Quantitative studies of microcirculatory structure and function. (in press, Circ. Res.).
RESEARCH,
7,351-361 (1974)
Pre- and Postcapillary Resistances in Cat Mesentery1s2 KITTY FRONEK AND BENJAMIN
W. ZWEIFACH
AMES-Bioengineering, University of California at San Diego, Lu JoNa, California 92037 Received March 6, 1973 The hemodynamic behavior of the isolated autoperfused intestinal loop (including mesenteric segment) of the cat was studied in terms of inflow and outflow pressures (BP and VP), arteriolar and venular pressures (P,, and P,), and blood flow (Q). The regional vascular resistance [RR = (BP - VP)/Q] was subdivided into a precapillary resistance [P,R= (BP - P.)/QJ, capillary resistance [CR= (P. - P,)/Q], and postcapillary resistance [P,R = (PV- VP)/Q]. With normal perfusion pressure it was found that: P,R = 68%, P,R = 20.5 %and CR=ll.S %.Twodifferentresponses were noted when perfusion pressure was lowered to 20mm Hg. In the first group, precapillary resistance fell significantly, whereas postcapillary resistance did not change. In the second group, the regional resistance increased due to the increase in precapillary resistance and a significantly greater increase in postcapillary resistance. The regulatory mechanism in the first group is based on local readjustment of the precapillary vasculature with no participation of postcapillary vessels. The second group of responses appears to be related to completely dilated vessels, where regulation is achieved by increasing postcapillary resistance in an attempt to maintain the capillary pressure and homeostatic balance.
INTRODUCTION Local adjustment in the terminal vascular bed is one of the principal mechanisms which determines fluid and nutrient exchange. To date the basic information on these regulatory mechanismshas beenalmost exclusively derived from indirect measurements. Folkow and co-workers (Folkow, Lundgren, and Wallentin, 1963)analyzedtherelationship between flow resistanceand regional blood volume in the intestine of the cat using the “isovolumetric” preparation. Johnson and Hanson (Johnson and Hanson, 1962) and subsequently Johnson (Johnson, 1960) utilized a combination of the isogravimetric method and direct cannulation of small arteries and veins in the isolated intestinal loop to calculate the pre- and postcapillary resistances. The autoregulatory response to an increase in perfusion pressure on the arteriolar diameter in the mesoappendix of the rat was documented by Baez (1968). Similarly, Johnson (Johnson, 1968)investigated arteriolar diameter changesin the cat mesentery of an isolated autoperfused intestinal loop with stepwise reduction of the perfusion pressure. In these studies, it was found that when perfusion pressureis reduced, there is a decreasein the pre- to postcapillary resistance ratio, which was interpreted to be a 1 This work was supported by a NIH Grant No. USPHS HL-10881. z Part of this work was presented at the 20th Annual Meeting of the Microcirculatory Atlantic City, New Jersey, April 9, 1972. 351 Copyright 0 1974 by Academic Press, Inc. All rights of reproduction Printed in Great Britain
in any form
reserved.
Society in
352
FRONEK AND ZWEIFACH
consequence of autoregulatory responses. Additional evidence suggested that this local regulatory processtakes place in vesselssmaller than 0.1 mm, and most likely in arterioles smaller than 50pm. In recent studies, Richardson and Zweifach (Richardson and Zweifach, 1970; Zweifach and Richardson, 1970) reported the micropressure results from arterioles and venules utilizing Wiederhielm’s (Wiederhielm et al., 1964) electronic servonulling pressure measurementmethod in the cat mesentery. This method was utilized to determine: (a) the pressure distribution in the microvessels as related to their size; (b) the pre- and postcapillary resistanceswith referenceto the size of the microvessels; and (c) the respectiveparticipation of vesselssmaller than 25 pm in the autoregulatory response. METHOD Experiments were performed in the mesentery attached to the isolated loop of 10 cats weighing on the average 2.5 kg under pentobarbital anesthesia (30 mg/kg). The isolated autoperfused intestinal loop was prepared according to the method described by Johnson (Johnson, 1960; Johnson and Hanson, 1962)and Selkurt et al. (Selkurt et al., 1958).A segmentof the terminal ileum, 25-50 g in weight, was exteriorized through a midline incision and isolated from the remainder of the intestine by severing all mesenteric connections, leaving only the arterial and venous lines intact. Denervation resulted during the process of dissecting and cannulating the artery and vein. After heparinization (1000 units/kg), the mesenteric artery was connected to the femoral supply artery via polyethylene tubing. A 1 in. piece of silastic tubing was inserted into the arterial inflow line. A fine screw clamp placed on the soft silastic tubing permitted reduction of the inflow pressure head in fine steps.In each experiment, the initial pressure was reduced to 100 mmHg, subsequently reduced by 20 mmHg steps until 20 mmHg was reached. The pressure changes were determined 60-90 set after each step of blood pressure reduction. The inflow arterial blood pressure was measureddistally from the clamp by a Statham pressure transducer P23Db. The terminal portion of the mesenteric vein was cannulated, and blood was returned via the femoral vein after passing through a drop counter and a peristaltic pump triggered by a photoelectric sensor. The right atrium could not be used as the phlebostatic level for the outflow pressure because the mesenteric veins are drained by the portal vein in which the pressure, under physiological conditions, varies from 12 to 16 cm H,O. Therefore, the venous pressure was adjusted to a level where additional lowering of the outflow reservoir tubing was not followed by a further increase in blood flow. This venous outflow pressure was assumedto be the control pressure throughout the experiment. It varied from animal to animal between47 cm HzO. This pressurewas measured by a Statham pressuretransducer P23BB, connected by a T-junction to the outflow line, and corrected by the pressuredrop due to the resistanceof the outflow tubing. The perfused segment of intestine and mesentery was placed on a heated platform in an intravital microscope and continuously irrigated with a solution containing 8.7 g/liter NaCl, 0.3 g/liter KCl, 0.3 g/liter CaC&, and 10 g/liter gelatin buffered with Na,CO,, and maintained at body temperature. Glass micropipettes with sharpened tips 3-5 pm were filled with 2 M NaCl and were inserted into selectedmicrovesselswith the help of a micromanipulator. Blood pressure
PRE- AND POSTCAPILLARY
RESISTANCES
353
in the vesselsof the mesentery was measured directly with the servonulling method developed by Wiederhielm et al. (1964) and modified by Intaglietta, Pawula, and Tompkins (1970). In these preparations, it was possible to measure simultaneously inflow and outflow pressures, arteriolar and venular pressures,and total blood flow. This data was utilized to calculate pre- and postcapillary resistancesand their ratio. The same flow was used to calcu!ate resistancesin the various segmentssince the same amount of blood must flow in and out of the preparation under steady state conditions. Johnson (Johnson, 1960; 1968)has shown that under conditions of stepwise reduction of inflow pressure, autoregulatory responseswere equally exhibited by the mesentery and the whole intestine tested with the isogravimetric technique as well as with the measurementsof arteriolar diameter changesin the mesenteryonly (Johnson, 1968). Therefore, it can be assumedthat flow changesin the mesentery reflect the corresponding changes in the intestinal wall. As a consequence,if regional resistance is taken to be the sum of resistances,then P,R + P,R + CR = RR. The resistances(Fig. 3) for any particular segment of mesenteric vasculature were calculated as follows: RR = (BP - VP)lQ,
(1)
where RR regional resistance BP inflow arterial blood pressure VP outflow venous blood pressure
Q outflow for the whole preparation P,R = (BP - P,)/Q
(2)
P,R = (P” - VP)/Q
(3)
CR = (Pa - PJQ,
(4)
where P,R precapillary resistance Pa blood pressure in given feeding arteriole P,R postcapillary resistance P, blood pressure in draining venule CR capillary resistance as calculated here represents the lumped resistancesfor
the vascular network of vesselsin the range of 6-20 pm on the arteriolar and venular side. Changes in resistance due to the BP manipulation were compared with respective control values and analyzed by Student’s t test and the limit for statistical significance was considered P co.05 or less. Diameters of all vesselsand their position in the terminal vascular bed were determined from photographs at microscopic magnification of 200-300x. Illumination was provided by a 100 W Xenon arc lamp. RESULTS The anatomy of selectedmicrovesselswhere the measurementshave been performed were either (a) continuously aligned paired arterioles and venules (Fig. 1) which are usually defined as arcuate vessels,although the artery and vein do not always servethe
354
FIG. 1. Microphotograph pipettes.
FRONEK AND ZWEIFACH
of paired arteriole and venule. Arrows indicate the position of micro-
samearea (Fraser and Wayland, 1972),or (b) vesselsin which one feeding arteriole and venule served to supply and to drain a well-defined region (Fig. 2). A total of 104 vesselsstudied represent the statistical population of the respective sites. The distribution of blood pressures and vascular resistances in arterioles and venules.
In view of differencesin systemic blood pressurein individual cats, the mean pressures in microvesseIsare expressedas percentageof the mean systemicblood pressure(Table I). Blood pressure in the small arterioles of approximately 20 pm in diameter is of the order of about 38 % of the mean arterial blood pressure, while in venules of the same diameter the pressure is approximately 25%. The pressure drop across true capillaries (exchange vessels) in the mesentery averaged 127; of the systemic mean arterial blood pressure or 14 cm H20. Venular pressuresin the mesentery were higher than venular pressuresin other vascular regions such as skin or skeletal muscle, because under normal conditions the splanchnic venous (portal) pressure is the highest in the organism. This is well documented by the results obtained in our laboratory from measurementsperformed on cat mesentery in situ (Zweifach, 1933).
D* (wn)
6.9 3
0 3
.-
11.5 3
5
6.52
5
5.5 5
3.35
4.42 25
1.82 25
126.7 6.2 26
46.2 3.29 17
14.63 1.23 17
Diameter of microvessels < 20 pm
4.1 38
38 2.32 17
2.06 25
Diameter of microvessels 50-21 ,um 53.7 132 67.0 37.6
23
6
Diameter of microvessels 100-51 pm 56.2 133 14.5 65
11.4 3
__32 2.16 14
29.0 2.3 22
18
2.13
26.8
751.1 1.13 14
36.4 2.16 22
18
2.7
65
(CAT MESENTERY)
26.4 2.47 14
25.0 1.42 22
18
1.21
19.8
--16.1 1.33 26
16.0 0.77 38
23
1.1
17.6
14.8 3.5 3
5.43 0.40 26
5.4 0.54 38
3.68 0.29 17
2.59 0.35 25
5
0.26
0.20 23
2.26
0.59 0.24 3
5.25
5.51 0.84 3
0.63 0.21 14
1.56 0.34 12
2
0.18
2.03
1.13 0.10 14
1.09 0.05 22
18
0.09
0.94
a Abbreviations: BP, mean arterial blood pressure; Pa, mean arteriolar pressure; P,, mean venular pressure; D., arteriolar diameter; D,, venulardiameter; SE, Standard error of mean; N, number of measurement. Resistances are expressed as mmHg/ml-i/min-l/lOO-l g of tissue. For details see text.
Number
SE
Average
Number
SE
Average
Number ~__
SE
Average
Average SE Number
p. (cm HA2
Diameter of microvessels < 100 ,~m 90 118 107 200
BP (cm HA3
1
CONTROLHEMODYNAMICDATA OF THE TERMINAL VASCULAR BED”
TABLE
-
3 2 3 E
B
1
2
$
3 3
5
P
356
FRONEK AND ZWEIFACH
FIG. 2. Microphotograph of an area with one feeding arteriole and a draining venule. Arrows indicate position of micropipettes.
Sixty-eight percent of RR is contributed by the precapillary vessels,i.e., vesselswith a diameter greater than 20 pm but still accessibleto intubation by the micropipette without interrupting flow (Fig. 3). There is no significant pressure difference between the group of arterioles from 100 to 50 pm, and the group from 50 to 21 pm, while the average pressure in the <20pm vesselsis significantly smaller. This finding suggeststhat the 50 to 21 pm vessel group is the primary determinant of P,R. 20.5 % of RR is generated by vesselsof the samesize, but on the venular side. The remainder constitutes the CR. This portion of vascular resistance is of composite origin and, due to the technique 60 40
ml4
20 i O
820 -A
iGT BP-‘Pa Q
I%+
21-50
51 - 100,(c,ons
P+ I
RR-Bp-vp
Q FIG. 3. Resistances distribution related to the size of the vessels. Resistances are expressed as per-
centage of total RR. Dotted columns summarize data from arterioles-plain columns from venules. Number of vesselsin each size range is the same as in Table I. For details see text.
PRE- AND POSTCAPILLARY
RESISTANCES
357
used (size of the micropipette), someparticipation of precapillary sphincters cannot be excluded. Resistance changes due to decrease ofperfusion pressure. Control data were determined in 22 paired vessels,feeding and draining the sameregion. Subsequently,BP wasreduced in a stepwisefashion as described in the methodical part. BP was reduced in four steps and repeated usually in two to four runs. A new run was initiated only after control pressure conditions were reestablished. The arteriolar and venular resistanceswere routinely calculated from measurementsin vesselswith a diameter of 25-15 pm. The changesin resistanceare summarized on Figs. 4 and 5. Results from six animals can be placed in a group (A, Fig. 4) characterized by the fall of RR with each decreaseof perfusion pressure, mainly asa consequenceof a decreaseP,R. When the perfusion pressure
20
40
INFLOW
60
a0
100
p mmHg
FIG. 4. Resistance changes in Group A due to decrease of the perfusion pressure. Resistances are expressed as cm H20/ml/min/100 g. RR, regional resistance; P,R, precapillary resistance, P,R, postcapillary resistance. (For details see text.)
was lowered from 100to 20 mmHg, RR fell from 7.2 f 0.62 (SE) cmH,O/ml/min/l00 g to 4.4 f 0.57 (SE). At the sametime P,R fell from 5 f 0.56 (SE) to 2.15 10.3 (SE) cm H,O/ml/min/lOO g. P,R changed very little, but showed a tendency to increase from 1.35 + 0.15 (SE)cm H,O/ml/min/lOO g to 2.16 + 0.5 (SE). This changeisnotstatistically significant. In the remainder of four animals-Group B, Fig. 5-RR increased as the perfusion pressure was lowered. This rise in resistancewas associatedwith a substantial increase in P,R’s. When BP was lowered progressively from 100 to 20 mmHg, RR rose from 7.17 f 0.4 (SE) cm H,O/ml/min/lOO g to 13.4 f 0.9 (SE). The increase in P,R from 4.4 f 0.15 (SE) cm H,O/ml/min/lOO g to 6.14 + 0.74 (SE) was significant (t = 6.776) from 1.46 f 0.18 (SE) to 4.83 f 0.46 (SE) cm H,O/ml/min/lOO g. If the sameresults are expressedas pre- to postcapillary resistanceratio, a decreasecan be observedfrom 3.78 to 1.34 in Group A and a similar drop from 3.23 to 1.29 in Group B. Even though the pre- to postcapillary resistance ratios in both groups are similar, the mechanism of
358
FRONEK AND ZWEIFACH
their changein the two groups is reversed: in A, the pre- to postcapillary resistanceratio decreasesdue to a decreasein P,R, and in group B, due to a predominant increase in
P,R. 14
10
a a
6
a
4
a”
2
+kT%++? INFLOW
100
P mmklg
FIG. 5. Resistance changes in Group B due to decrease of the perfusion pressure. Description same as in Fig. 4.
C B y=xo544+J.i47
0
4
8
12
I6
20
Pv cmHLO
24
28
32
36
0
4
0
12
I6
20
24
2.3
32
36
Pv cmHrO
FIG. 6. Flow to venular pressure relationship due to decrease of perfusion pressure. Flows are expressed in ml/min/lOCl g of tissue, venular pressures in cm H,O. Right side summarizes data for Group A, left for Group B.
PRE- AND POSTCAPILLARY
RESISTANCES
359
The changesin postcapillary vesselsduring reduction of BP were analyzed in terms of pressure flow relationship. From the comparison of the regression lines and their intercepts for both groups (Fig. 6), it can be seenthat in Group B the venular pressures are higher than in Group A for the sameflow rates. No significant difference was found between both regression lines. Vesselsselected for this correlation were of the same diameter. DISCUSSION Direct pressure measurementsobtained in conjunction with intravital microscopy, and the subsequent calculation of the respective resistances,provides a better understanding of the hemodynamics of the terminal vasculature. The first significant pressure drop under steady-rate conditions occurs on the arteriolar side in the group of vessels from 100 to 50 pm diam. A second large pressure drop is in vesselswhich range from 20 pm diam to true capillaries. This pressurerelationship is different from the one found in skeletal muscle (Fronek and Zweifach, 1973),where the largest pressuredrop occurs in arterioles between 35 and 10 pm diam. The averagevenular pressurein the smallest veins is 24 mmHg which is the nearest value to the capillary pressure. In comparison with the results reported by Richardson and Zweifach (1970) in the sameexperimental arrangement, the present average value is 4 mmHg lower. Johnson, by using the isogravimetric method in the autoperfused isolated intestinal loop (Johnson and Hanson, 1962), obtained even lower values, and questioned the use of the isogravimetric technique in this type of preparation. It should be noted, however, that he assumed, on the basis of plasma protein content, an effective capillary pressure of 20-21 mmHg (Johnson and Hanson, 1962). The presented venular pressuresare in agreementwith in situ data from the cat mesentery(Zweifach, 1973). In the present experiments, the averagepre- to postcapillary resistanceratio of 3.5 is smaller than that obtained for the hind limb by Pappenheimer and Soto-Rivera (Pappenheimer and Soto-Rivera, 1948)and by Mellander (Mellander, 1960).In comparing our data and previous estimates, several factors should be considered. Pappenheimer and Soto-Rivera (Pappenheimer and Soto-Rivera, 1948) derived the RR ratio from measurements on the hind limb, while our results were obtained from the isolated intestinal loop. As has been pointed out, the venous pressuresin the splanchnic region are higher than in the skeletal muscle. Secondly, the pre- and postcapillary resistances were calculated in Pappenheimer and Soto-Rivera’s experiments on the basis of mean capillary pressure measurements,whereas our pressure data were obtained from preand postcapillary vesselsdirectly, so that the resistance values are necessarily slightly lower. For the same reason, the resistance between the arteriolar and venular site of measurements--capillary reistance-is slightly larger. When the perfusion pressure is lowered from 100 to 20 mmHg, the pre- and postcapillary ratio drops almost to 1, indicating that the pre- and postcapillary resistances became equal, while CR does not change. As has been shown, such a change in the resistanceratio either can be due to a decreaseof P,R (as found in Group A), or due to a predominant increase of P,R (Group B). The regulatory mechanism responsible for the type of adjustment characteristic of Group A is similar to that reported by Johnson (Johnson, 1960),and has been explained on the basis of the metabolic or the myogenic
360
FRONEK AND ZWEIFACH
theory, or both (Johnson, 1960; Johnson and Hanson, 1962). In such animals, local regulation could be achieved by readjustments of the precapillary vasculature with essentially no participation of the venular segment. The reaction pattern in this group does not support Johnson’s findings on venoconstriction. It should be pointed out that Johnson’s results are basedon the cannulation of small veins while this report describes measurementsin veins of 25 ,nmdiam, not accessibleby cannulation. It is possible that the effect of distally located venoconstriction could be mitigated to some extent by a sufficiently compliant capacitance system located between the site of constriction and the site of micropuncture of the venule. On the other hand in Group B, the response to decreasing perfusion pressure was characterized by a substantially greater increase in venular resistance.The explanation for this type of response is not clear. It is possible that in these casesthe precapillary resistancevesselscan no longer undergo physiologic dilatation, so that an increase in P,R, with a reduction in BP, balances capillary pressure and fluid movement equilibrium. Support of this hypothesis is provided by the observation that while the RR is the same in both groups, the P,R in Group B at the beginning of the run was significantly lower (P ~0.05). Furthermore, in these cases,reactive hyperemia was substantially reduced after the release of occlusion (43 vs 93%). There is no simple explanation to the increasing P,R in the nonautoregulating group. It is possible that thin-walled vesselsunder low tension, passively decreasein diameter, when perfusion pressure is lowered (Nagle et al., 1968).This factor does not seemto be decisiveunder thesecircumstancessincethe venular pressurescorresponding to similar flows are higher in the nonautoregulating group (Fig. 6). If the increase in P,R were a function of simple recoil effect, the pressuresin Group B would be lower or at least equal to pressuresmeasuredat corresponding flow rates in the autoregulating group. The participation of extrinsic nervous influence can be ruled out in view of the surgical isolation of the mesentery. On the other hand, it is possible that in nonautoregulating preparations, the arteriovenous reflex is quantitatively more significant (Hanson and Johnson, 1962). The interplay between pre- and postcapillary resistancesrepresentsan active physiological mechanism which is instrumental in maintaining capillary pressure and fluid equilibrium. Summarizing the responsesto stepwise reduction of perfusion pressure, it seemsthat this can be achieved either by decreasing the precapillary resistance or increasing the postcapillary resistance-in both cases decreasing the pre- to postcapillary resistance ratio. REFERENCES 1.
S. (1968). Bayliss response in the microcirculation. Fed. Proc. Fed. Amer. Sot. Exp. Biol. 27, 141%1415. 2. FOLKOW,E., LUNDGREN,O., AND WALLENTIN,I. (1963). Studies on the relationship between flow resistance, capillary filtration coefficient and regional blood volume in the intestine of the cat. Acta &and. Physiol. 57, 270-283. 3. FRASHER,W. G., JR., AND WAYLAND, H. (1972). A repeating modular organization of the microcirculation of cat mesentery. Microuasc. Res. 4, 62-76. 4. FRONEK, K., AND ZWEIFACH, B. W. (1973). Comparison of the effect of Papaverine on terminal vasculature of the cat mesentery and skeletal muscle. Presented at the 21st Annual Meeting of the Microcirculatory Society, Atlantic City, April 14, 1973. BAEZ,
PRE- AND POSTCAPILLARY
RESISTANCES
361
5. INTAGLIETIA, M., PAWULA, R. F., AND TOMPKINS,W. R. (1970). Pressure measurements in the mammalian microvasculature. Microvasc. Res. 2,212-220. 6. HANSON,K. M., AND JOHNSON,P. C. (1962). Evidence for local arterio-venous reflex in intestine. J. Appl. Physiol. 17, 509-513. 7. JOHNSON,P. C. (1960). Autoregulation of intestinal blood flow. Amer. J. Physiol. 199,311-318. 8. JOHNSON,P. C., AND HANSON,K. M. (1962). Effect of arterial pressure on arterial and venous resistance of intestine. J. Appl. Physiol. 17,503-508. 9. JOHNSON,P. C. (1968). Autoregulatory response of cat mesenteric arterioles measured in vivo. Circ. Res. 22,199-212. 10. MELLANDER,J. (1960). Comparative studies on the adrenergic neurohormonal control of resistance and capacitance blood vessels in the cat. Acta &and. Physiol. 50,176-186. 11. NAGLE, F. J., SCOTT,J. B., SWINDALE,B. T., AND HADDY, F. J. (1968). Venous resistances in skeletal muscle and skin during local blood flow regulation. Amer. J. Physioi. 214,885-891. 12. PAPPENHEIMER, J. R., AND SOTO-RIVERA,A. (1948). Effective osmotic pressure of the plasma protein and their quantities associated with the capillary circulation in the hind limb of cats and dogs. Amer. J. Physiol. 152,471491. 13. RICHARDSON,D. R., AND ZWEIFACH,B. W. (1970). Pressure relationship in the macro and microcirculation of the mesentety. Microvasc. Res. 2,474-487. 14. SELKURT,E. E., SCIBETTA,M. P., AND CULL, B. E. (1958). Hemodynamics of intestinal circulation. Circ. Res. 6,92-99. 15. WIEDERHIELM,C. A., WOODBURY,J. W., KIRK, S., AND RUSHMER,R. F. L. (1964). Pulsatile pressure in microcirculation of the frog’s mesentery. Amer. J. Physiol. 207,173-176. 16. ZWEIFACH,B. W., AND RICHARDSON, D. R. (1970). Microcirculatory adjustments of pressure in the mesentery. 6th Eur. Co& Microcirculation, Aalborg, 248-253. 17. ZWEIFACH,B. W. (1933). Quantitative studies of microcirculatory structure and function. (in press, Circ. Res.).