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Regional hydrostatic pressure differences: Relation to spatial variations in arteriolar cell flux and tone
MICROVASCULAR
RESEARCH
0,
112-117 (1992)
BRIEF COMMUNICATION Regional
Hydrostatic Variations
Pressure Differences: Relation to Spatial in Arteriolar Cell Flux and Tone
TERRENCE Department of Physiology,
E. SWEENEY AND ROBERT W. GORE
College of Medicine,
The Vniversiry of Arizona,
Tucson, Arizona
85724
Received May 13, 1991
INTRODUCTION Spatial heterogeneity of blood flow and flow reserve are common to a number of tissues, including striated muscle (Hargreaves et al., 1990), heart (Austin et al., 1990), and kidney (Brezis et al., 1984). Although flow heterogeneity has been implicated as an important factor in vulnerability to ischemia (Austin et al., 1990) and is a common focus of study, its causes and physiological roles remain unclear. Significant spatial variations in resting diameter and red cell flux were observed recently in arterioles that control capillary perfusion in the hamster cremaster muscle (Sweeney and Sarelius, 1989, 1990), and in the capillary networks supplied by these vessels. These regional differences in vessel diameter and tissue perfusion were abolished during maximal vasodilation. A possible explanation for this unequal tissue flow distribution is that metabolic demand varies across the tissue and that blood flow is altered regionally to match supply to local demand. However, determination of the muscle fiber type composition of the cremaster showed that the oxidative capacity of this muscle was spatially uniform (Sarelius et al., 1983; Sarelius, personal communication), making it unlikely that regional differences in tissue demand drive the observed blood flow heterogeneity. An alternative explanation is that hemodynamics, precapillary exchange, and the topography of the microvascular network result in spatial inequalities in the parameters that determine blood supply and that arterioles respond to local supply conditions to match capillary perfusion to tissue demand for blood flow. Evidence of precapillary oxygen losses (Duling and Berne, 1970) indicates that regional differences in oxygen supply are likely. Furthermore, a significant decline in intravascular pressure has been observed along the length of single microvessels (Davis et al., 1986). In this report, we have quantified regional differences in hydrostatic pressure in the resting cremaster muscle to determine the spatial relationship between feeding pressure and regional variations in arteriolar tone and blood flow. We measured intravascular pressure in the central feed arteriole of the hamster cre112 @X6-2862/92 $3.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form resewed. Printed in U.S.A.
BRIEF COMMUNICATION
113
master muscle at the two points where arterioles branch to supply the regions of the cremaster microvasculature previously shown to have significant differences in cell flow and tone (Sweeney and Sarelius, 1990). Existing data on the response of arterioles to step changes in pressure (Meininger et al., 1987) were used to help determine whether the disparity in feeding pressures at the two tissue sites was sufficient to explain the reported regional variations in resting arteriolar cell flux and tone (Sweeney and Sarelius, 1990), or whether inequalities in other determinants of arteriolar tone were necessary to account for the reported flow heterogeneity. MATERIALS
AND METHODS
Preparation. Male golden hamsters (Charles River) (115-147 g) were used. Each animal was anesthetized with sodium pentobarbital (Nembutal, Abbot Laboratories; 70 mg/kg, ip). The trachea was intubated to ensure a patent airway, and the left femoral artery and vein were cannulated. Systemic pressure was recorded with a Spectramed (Model TNF-R) pressure transducer via the arterial catheter. Saline containing 10 mg/ml sodium pentobarbital was infused (0.56 ml/hr) via the venous catheter to maintain anesthesia and compensate for renal and respiratory fluid losses. Deep body temperature was maintained at 37-38” by a thermostated warming coil. The right cremaster muscle was prepared and superfused as described in detail elsewhere (Sarelius, 1986). The preparation was maintained at 34 t 0.5” via the warmed, gassedsuperfusate (95% NZ, 5% COZ, bicarbonate buffered physiological salt solution, pH 7.37). During dissection, care was taken to cut and lay out the muscle in a manner that presented the same general arteriolar network geometry. The tissue was allowed to stabilize for 1 hr before data were taken. In all preparations, the presence of vasoactive tone was confirmed in two to three randomly selected arterioles by observation of brisk dilation in response to local application of 1O-4 M adenosine. Arteriolar oxygen sensitivity was demonstrated by observing vasoconstriction in response to transient equilibration of the suffusate with gas containing 10% oxygen. The age and origin of the hamsters used and the investigator who prepared the animals were the same in these experiments as in the experiments in which spatial differences in arteriolar tone and cell flux were first observed (Sweeney and Sarelius, 1989, 1990). Micropressure measurements. An IPM Model 4A servo-micropressure system was used to measure arteriolar pressures in the cremaster. Pressures were measured through beveled, glass micropipets (l-2 pm o.d.). Each pipet was calibrated before use. Pressure measurements were accepted if they satisfied the following criteria: (1) the recording was stable and free of noise; (2) the pressure record was insensitive to slight manipulation of the micropipet within the lumen of a vessel; (3) the pipet calibration was linear with a slope of 1 over the range O160 mm Hg; (4) the pulse recorded in a microvessel was synchronous with that of the arterial pressure record; (5) the pressure signal returned to zero upon removal of the pipet from the vessel. Experimental protocol. In 10 hamsters, paired measurements of intravascular pressure and arteriolar diameter were made at two specified sites along a central feed arteriole in the resting cremaster muscle. To minimize anatomical variations,
114
BRIEF COMMUNICATION
each measurement site was carefully chosen and identified on a map of the microvascular network of each hamster. The sites were chosen to be adjacent to the network sites observed by Sweeney and Sarelius (1990), as shown in their Fig. 1. The order in which measurements were performed was chosen randomly. In a subset of the experimental group, measurements were made in both the resting tissue and during maximal vasodilation produced by the topical application of approximately 1 ml of 10P4 M adenosine in saline. Arteriolar and systemic pressure data were recorded on a Gould 260 chart recorder and on a DATA 6000 digital waveform analyzer. Arteriolar diameter was measured directly off the monitor during videotape playback, using a videotaped stage micrometer for reference. At the completion of each experiment, the entire cremaster network was recorded and a detailed map of the arteriolar network was drawn. Data analysis. Each pressure record was digitally averaged over its collection period (approximately 4 min) and arteriolar pressure was normalized against systemic pressure. Mean values are presented ? 1 standard error. Group means were compared using paired t tests (Snedecor and Cochran, 1967). Only complete pairs were used in statistical analyses. In instances of missing data points, mean values from complete pairs (not reported) closely approximated the respective means from the whole data sets. Significance was assessedat the 95% confidence level.
RESULTS Representative tracings of simultaneously recorded arterial and arteriolar pressures are shown in Fig. 1. In the experiment from which these traces were obtained, mean arterial pressure was a constant 91 + 1 mm Hg during all micropressure measurements. Figure 1 shows that in this resting muscle, the mean pressure in the central feed arteriole at the more distal site II (33.5 mm Hg, Fig. 1, left) was approximately 4 mm Hg less than that at site I (Fig. 1, center). The right side of Fig. 1 shows that when arteriolar dilation was induced by topical application of 10e4 M adenosine to the cremaster, the arteriolar pressure at site I dropped from its mean resting value of 37.7 mm Hg to a minimum of 27.6 mm Hg. The effect of arteriolar dilation on the pressure at site II was not recorded in this experiment. Mean pressure data are presented in Fig. 2, expressed as a percentage of arterial pressure. Arterial pressure averaged 82.4 + 3.3 mm Hg (n = 10) during measurements at site I and 81.8 + 3.1 mm Hg (n = 10) during measurements at site II. The distance between the measurement locations averaged 7430 + 720 pm (n = 10). Over this distance, the mean resting feed arteriole pressure dropped nearly 4 mm Hg, from 36.8 + 2.5 mm Hg at site I (n = 10) to 33.0 ? 2.0 mm Hg at site II (n = 10). Although the difference in absolute pressure was only marginally significant (P = 0.055), when arteriolar pressure was normalized against arterial pressure (Fig. 2), the drop in pressure between the two sites was strongly significant (P < 0.01). Vasodilation significantly reduced mean arteriolar pressure at both sites along the feed arteriole by over 10 mm Hg and reduced the pressure difference between the two sites. The mean pressures recorded during vasodilation, 25.7 + 1.8 mm Hg at site I (n = 6) and 23.5 + 2.8 mm Hg at site II (n = 3),
115
BRIEF COMMUNICATION Femoral Artery 160
120
60
1Q-4 M Adenosine
Feed Arteriole
Site I
Seconds Digitized pressure tracings from the two measurement sites along the central feed arteriole in one tissue, shown with simultaneous recordings of systemic pressure. Left trace: recording from site II of the arteriole, in the resting muscle. Center trace: recording from site I of the arteriole; resting muscle. Right trace: recording from site I of the arteriole upon topical application of 1 ml 10m4M adenosine. See text for details.
F
40 30 20 10 OSite I ADO Site II Rest Site II ADO Site I Rest 2. Mean central feed arteriole pressures, normalized against arterial pressure. Data from the resting muscle are shown on the left side (n = 10, each site). The right two bars show mean values after the topical application of 1 ml 10m4M adenosine (site I, n = 6; site II, n = 3). Solid bars: site I data. Hatched bars: site II data. Bars represent 1 standard error. *Significantly different from resting value at site I. FIG.
116
BRIEF COMMUNICATION
were not significantly different, even when normalized against arterial pressure. This may be due to the small number of observations made during vasodilation. The diameter of the central feed arteriole did not differ significantly between the two sites under either experimental condition. In the resting muscle it averaged 74.0 + 4.1 pm at site I (n = 10) and 68.8 + 4.5 pm at site II (n = 8). During the application of 1O-4 M adenosine, central feed arteriole diameter averaged 90.0 2 8.4 pm at site I (n = 6) and 81.1 + 4.6 pm at site II (n = 3).
DISCUSSION Intravascular hydrostatic pressure was measured at two sites along the hamster cremaster central feed arteriole to quantify the relationship between feeding pressure and arteriolar tone and to assessthe contribution of regional pressure differences to spatial heterogeneity in blood flow distribution. The data show that in the resting muscle, two tissue sites that differ greatly in arteriolar tone and cell flux (Sweeney and Sarelius, 1990) are exposed to significantly different feeding pressures. Feed pressure was higher in the region shown previously (Sweeney and Sarelius, 1990) to have greater arteriolar tone and lower resting cell flux. During maximal vasodilation, when tone and cell flux were spatially uniform (Sweeney and Sarelius, 1990), local perfusion pressure decreased and the regional difference in pressure was reduced. Although the observed regional pressure difference qualitatively supports a hypothesis of myogenically induced regional flow disparity, it is clear from the magnitude of the pressure difference that other spatial inequalities must also be involved in producing blood flow heterogeneity. Pressure measurements showed that feeding pressure was 4 mm Hg higher at site I than at site II of the central feed arteriole. In a previous study, cell flux in the arterioles that control capillary perfusion was shown to be 42% less at the higher pressure site I than at site II (Sweeney and Sarelius, 1990). The pressure and cell flux data are thus qualitatively consistent with an hypothesis of myogenically induced flow heterogeneity. However, data of Meininger et al. (1987) indicate that a regional difference of nearly 20 mm Hg in feed arteriolar pressure would be required to achieve a 42% disparity in blood flow via myogenic control of precapillary tone. Thus, even though significant flow reductions occur following pressure increases as small as 10 mm Hg (Meininger et al., 1987), it is clear from the present data that spatial variations in hamster cremaster cell flux are not simply the result of myogenic autoregulation. The potential consequences of blood flow heterogeneity are of sufficient concern that continued exploration of the mechanism involved is warranted. It appears likely that regional differences in blood flow distribution are caused by a combination of factors that may modulate one another. For example, intravascular pressure has been shown to alter the sensitivity of vessels to vasoconstrictor agonists (Tallarida et al., 1974; Lombard et al., 1990). Thus the regional response to circulating agonists may differ as a result of spatial variations in transmural pressure. Second, oxygen levels and the concentrations of cellular metabolites have been shown to modulate neural and local regulators of arteriolar tone (Marshall, 1982; Boegehold and Johnson, 1988; McGillivray-Anderson and Faber, 1990). Finally, it has been shown recently that EDRF inhibits myogenic tone
BRIEF
COMMUNICATION
117
(Griffith and Edwards, 1990). It is hoped that further experimentation will sort out the consequences of these complex interactions on blood flow heterogeneity. ACKNOWLEDGMENTS The authors thank Mr. John S. Rozum for his excellent technical assistance. This work was supported by Grants HL13437, HL07506, and HL07249 from the National Heart, Lung and Blood Institute. Preliminary results of this work were published previously in abstract form (FASEB. J. (1989) 3, A1402).
REFERENCES AUSTIN, R. E., ALDEA, G. S., COGGINS,D. L., FLYNN, A. E., AND HOFFMAN, J. I. E. (1990). Profound
spatial heterogeneity of coronary reserve: Discordance between patterns of resting and maximal myocardial blood flow. Circ. Res. 67, 319-331. BOEGEHOLD, M. A., AND JOHNSON, P. C. (1988). Periarteriolar and tissue PO2 during sympathetic escape in skeletal muscle. Am. J. Physiof. 254, H929-H936. BREZIS, M., ROSEN, S., SILVA, P., AND EPSTEIN, F. H. (1984). Renal ischemia: A new perspective. Kidney Int. 26, 375-383. DAVIS, M. J., FERRER, P. N., AND GORE, R. W. (1986). Vascular anatomy and hydrostatic pressure in the hamster cheek pouch. Am. J. Physiol. 250, H291-H303. DULING, B. R., AND BERNE, R. M. (1970). Longitudinal gradients in periarteriolar oxygen tension: A possible mechanism for the participation of oxygen in local regulation of blood flow. Circ. Res.
27, 669-678. GRIFFITH, T. M., AND EDWARDS, D. H. (1990). Myogenic autoregulation of flow may be inversely related to endothelium-derived relaxing factor activity. Am. J. Physiol. 258, H1171-H1180. HARGREAVES,D., EGGINTON, S., AND HUDLICK~~, 0. (1990). Changes in capillary perfusion induced by different patterns of activity in rat skeletal muscle. Microvasc. Res. 40, 14-28. LOMBARD, J. H., ESKINDER, H., KAUSER, K., OSBORN, J. L., AND HARDER, D. R. (1990). Enhanced norepinephrine sensitivity in renal arteries at elevated transmural pressure. Am. J. Physiol. 259,
H29-H33. MARSHALL, J. M. (1982). The influence of the sympathetic nervous system on individual vessels of the microcirculation of skeletal muscle of the rat. J. Physiol. 332, 169-186. MCGILLIVRAY-ANDERSON, K. M., AND FABER, J. E. (1990). Effect of acidosis on contraction of mi-
crovascular smooth muscle by or- and a,-adrenoceptors: Implications for neural and metabolic regulation. Circ. Res. 66, 1643-1657. MEININGER, G. A., MACK, C. A., FEHR, K. L., AND BOHLEN, H. G. (1987). Myogenic regulation overrides local metabolic control in resting rat skeletal muscle. Circ. Res. 60, 861-870. SARELIUS,I. H. (1986). Cell flow path influences transit time through striated muscle capillaries. Am. J. Physiol. 250, H899-H907. SARELIUS, I. H., MAXWELL, L. C., GRAY, S. D., AND DULING, B. R. (1983). Capillarity and fiber types in the cremaster muscle of rat and hamster. Am. J. Physiol. 245, H368-H374. SNEDECOR,G. W., AND C~CHRAN, W. G. (1967). “Statistical Methods,” 6th ed., Iowa State Univ. Press, Ames, IA. SWEENEY,T. E., AND SARELIUS,I. H. (1989). Arteriolar control of capillary cell flow in striated muscle. Circ. Res. 64, 112-120. SWEENEY, T. E., AND SARELIUS, I. H. (1990). Spatial heterogeneity in striated muscle arteriolar tone, cell flow and capillarity. Am. J. Physiol. 259, H124-H136. TALLARIDA, R. J., SEVY, R. W., HARAKAL, C., BENDRICK, J., AND FAUST, R. (1974). The effect of preload on the dissociation constant of norepinephrine in isolated strips of rabbit thoracic aorta. Arch. Znt. Pharmacodyn.
210, 67-74.
RESEARCH
0,
112-117 (1992)
BRIEF COMMUNICATION Regional
Hydrostatic Variations
Pressure Differences: Relation to Spatial in Arteriolar Cell Flux and Tone
TERRENCE Department of Physiology,
E. SWEENEY AND ROBERT W. GORE
College of Medicine,
The Vniversiry of Arizona,
Tucson, Arizona
85724
Received May 13, 1991
INTRODUCTION Spatial heterogeneity of blood flow and flow reserve are common to a number of tissues, including striated muscle (Hargreaves et al., 1990), heart (Austin et al., 1990), and kidney (Brezis et al., 1984). Although flow heterogeneity has been implicated as an important factor in vulnerability to ischemia (Austin et al., 1990) and is a common focus of study, its causes and physiological roles remain unclear. Significant spatial variations in resting diameter and red cell flux were observed recently in arterioles that control capillary perfusion in the hamster cremaster muscle (Sweeney and Sarelius, 1989, 1990), and in the capillary networks supplied by these vessels. These regional differences in vessel diameter and tissue perfusion were abolished during maximal vasodilation. A possible explanation for this unequal tissue flow distribution is that metabolic demand varies across the tissue and that blood flow is altered regionally to match supply to local demand. However, determination of the muscle fiber type composition of the cremaster showed that the oxidative capacity of this muscle was spatially uniform (Sarelius et al., 1983; Sarelius, personal communication), making it unlikely that regional differences in tissue demand drive the observed blood flow heterogeneity. An alternative explanation is that hemodynamics, precapillary exchange, and the topography of the microvascular network result in spatial inequalities in the parameters that determine blood supply and that arterioles respond to local supply conditions to match capillary perfusion to tissue demand for blood flow. Evidence of precapillary oxygen losses (Duling and Berne, 1970) indicates that regional differences in oxygen supply are likely. Furthermore, a significant decline in intravascular pressure has been observed along the length of single microvessels (Davis et al., 1986). In this report, we have quantified regional differences in hydrostatic pressure in the resting cremaster muscle to determine the spatial relationship between feeding pressure and regional variations in arteriolar tone and blood flow. We measured intravascular pressure in the central feed arteriole of the hamster cre112 @X6-2862/92 $3.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form resewed. Printed in U.S.A.
BRIEF COMMUNICATION
113
master muscle at the two points where arterioles branch to supply the regions of the cremaster microvasculature previously shown to have significant differences in cell flow and tone (Sweeney and Sarelius, 1990). Existing data on the response of arterioles to step changes in pressure (Meininger et al., 1987) were used to help determine whether the disparity in feeding pressures at the two tissue sites was sufficient to explain the reported regional variations in resting arteriolar cell flux and tone (Sweeney and Sarelius, 1990), or whether inequalities in other determinants of arteriolar tone were necessary to account for the reported flow heterogeneity. MATERIALS
AND METHODS
Preparation. Male golden hamsters (Charles River) (115-147 g) were used. Each animal was anesthetized with sodium pentobarbital (Nembutal, Abbot Laboratories; 70 mg/kg, ip). The trachea was intubated to ensure a patent airway, and the left femoral artery and vein were cannulated. Systemic pressure was recorded with a Spectramed (Model TNF-R) pressure transducer via the arterial catheter. Saline containing 10 mg/ml sodium pentobarbital was infused (0.56 ml/hr) via the venous catheter to maintain anesthesia and compensate for renal and respiratory fluid losses. Deep body temperature was maintained at 37-38” by a thermostated warming coil. The right cremaster muscle was prepared and superfused as described in detail elsewhere (Sarelius, 1986). The preparation was maintained at 34 t 0.5” via the warmed, gassedsuperfusate (95% NZ, 5% COZ, bicarbonate buffered physiological salt solution, pH 7.37). During dissection, care was taken to cut and lay out the muscle in a manner that presented the same general arteriolar network geometry. The tissue was allowed to stabilize for 1 hr before data were taken. In all preparations, the presence of vasoactive tone was confirmed in two to three randomly selected arterioles by observation of brisk dilation in response to local application of 1O-4 M adenosine. Arteriolar oxygen sensitivity was demonstrated by observing vasoconstriction in response to transient equilibration of the suffusate with gas containing 10% oxygen. The age and origin of the hamsters used and the investigator who prepared the animals were the same in these experiments as in the experiments in which spatial differences in arteriolar tone and cell flux were first observed (Sweeney and Sarelius, 1989, 1990). Micropressure measurements. An IPM Model 4A servo-micropressure system was used to measure arteriolar pressures in the cremaster. Pressures were measured through beveled, glass micropipets (l-2 pm o.d.). Each pipet was calibrated before use. Pressure measurements were accepted if they satisfied the following criteria: (1) the recording was stable and free of noise; (2) the pressure record was insensitive to slight manipulation of the micropipet within the lumen of a vessel; (3) the pipet calibration was linear with a slope of 1 over the range O160 mm Hg; (4) the pulse recorded in a microvessel was synchronous with that of the arterial pressure record; (5) the pressure signal returned to zero upon removal of the pipet from the vessel. Experimental protocol. In 10 hamsters, paired measurements of intravascular pressure and arteriolar diameter were made at two specified sites along a central feed arteriole in the resting cremaster muscle. To minimize anatomical variations,
114
BRIEF COMMUNICATION
each measurement site was carefully chosen and identified on a map of the microvascular network of each hamster. The sites were chosen to be adjacent to the network sites observed by Sweeney and Sarelius (1990), as shown in their Fig. 1. The order in which measurements were performed was chosen randomly. In a subset of the experimental group, measurements were made in both the resting tissue and during maximal vasodilation produced by the topical application of approximately 1 ml of 10P4 M adenosine in saline. Arteriolar and systemic pressure data were recorded on a Gould 260 chart recorder and on a DATA 6000 digital waveform analyzer. Arteriolar diameter was measured directly off the monitor during videotape playback, using a videotaped stage micrometer for reference. At the completion of each experiment, the entire cremaster network was recorded and a detailed map of the arteriolar network was drawn. Data analysis. Each pressure record was digitally averaged over its collection period (approximately 4 min) and arteriolar pressure was normalized against systemic pressure. Mean values are presented ? 1 standard error. Group means were compared using paired t tests (Snedecor and Cochran, 1967). Only complete pairs were used in statistical analyses. In instances of missing data points, mean values from complete pairs (not reported) closely approximated the respective means from the whole data sets. Significance was assessedat the 95% confidence level.
RESULTS Representative tracings of simultaneously recorded arterial and arteriolar pressures are shown in Fig. 1. In the experiment from which these traces were obtained, mean arterial pressure was a constant 91 + 1 mm Hg during all micropressure measurements. Figure 1 shows that in this resting muscle, the mean pressure in the central feed arteriole at the more distal site II (33.5 mm Hg, Fig. 1, left) was approximately 4 mm Hg less than that at site I (Fig. 1, center). The right side of Fig. 1 shows that when arteriolar dilation was induced by topical application of 10e4 M adenosine to the cremaster, the arteriolar pressure at site I dropped from its mean resting value of 37.7 mm Hg to a minimum of 27.6 mm Hg. The effect of arteriolar dilation on the pressure at site II was not recorded in this experiment. Mean pressure data are presented in Fig. 2, expressed as a percentage of arterial pressure. Arterial pressure averaged 82.4 + 3.3 mm Hg (n = 10) during measurements at site I and 81.8 + 3.1 mm Hg (n = 10) during measurements at site II. The distance between the measurement locations averaged 7430 + 720 pm (n = 10). Over this distance, the mean resting feed arteriole pressure dropped nearly 4 mm Hg, from 36.8 + 2.5 mm Hg at site I (n = 10) to 33.0 ? 2.0 mm Hg at site II (n = 10). Although the difference in absolute pressure was only marginally significant (P = 0.055), when arteriolar pressure was normalized against arterial pressure (Fig. 2), the drop in pressure between the two sites was strongly significant (P < 0.01). Vasodilation significantly reduced mean arteriolar pressure at both sites along the feed arteriole by over 10 mm Hg and reduced the pressure difference between the two sites. The mean pressures recorded during vasodilation, 25.7 + 1.8 mm Hg at site I (n = 6) and 23.5 + 2.8 mm Hg at site II (n = 3),
115
BRIEF COMMUNICATION Femoral Artery 160
120
60
1Q-4 M Adenosine
Feed Arteriole
Site I
Seconds Digitized pressure tracings from the two measurement sites along the central feed arteriole in one tissue, shown with simultaneous recordings of systemic pressure. Left trace: recording from site II of the arteriole, in the resting muscle. Center trace: recording from site I of the arteriole; resting muscle. Right trace: recording from site I of the arteriole upon topical application of 1 ml 10m4M adenosine. See text for details.
F
40 30 20 10 OSite I ADO Site II Rest Site II ADO Site I Rest 2. Mean central feed arteriole pressures, normalized against arterial pressure. Data from the resting muscle are shown on the left side (n = 10, each site). The right two bars show mean values after the topical application of 1 ml 10m4M adenosine (site I, n = 6; site II, n = 3). Solid bars: site I data. Hatched bars: site II data. Bars represent 1 standard error. *Significantly different from resting value at site I. FIG.
116
BRIEF COMMUNICATION
were not significantly different, even when normalized against arterial pressure. This may be due to the small number of observations made during vasodilation. The diameter of the central feed arteriole did not differ significantly between the two sites under either experimental condition. In the resting muscle it averaged 74.0 + 4.1 pm at site I (n = 10) and 68.8 + 4.5 pm at site II (n = 8). During the application of 1O-4 M adenosine, central feed arteriole diameter averaged 90.0 2 8.4 pm at site I (n = 6) and 81.1 + 4.6 pm at site II (n = 3).
DISCUSSION Intravascular hydrostatic pressure was measured at two sites along the hamster cremaster central feed arteriole to quantify the relationship between feeding pressure and arteriolar tone and to assessthe contribution of regional pressure differences to spatial heterogeneity in blood flow distribution. The data show that in the resting muscle, two tissue sites that differ greatly in arteriolar tone and cell flux (Sweeney and Sarelius, 1990) are exposed to significantly different feeding pressures. Feed pressure was higher in the region shown previously (Sweeney and Sarelius, 1990) to have greater arteriolar tone and lower resting cell flux. During maximal vasodilation, when tone and cell flux were spatially uniform (Sweeney and Sarelius, 1990), local perfusion pressure decreased and the regional difference in pressure was reduced. Although the observed regional pressure difference qualitatively supports a hypothesis of myogenically induced regional flow disparity, it is clear from the magnitude of the pressure difference that other spatial inequalities must also be involved in producing blood flow heterogeneity. Pressure measurements showed that feeding pressure was 4 mm Hg higher at site I than at site II of the central feed arteriole. In a previous study, cell flux in the arterioles that control capillary perfusion was shown to be 42% less at the higher pressure site I than at site II (Sweeney and Sarelius, 1990). The pressure and cell flux data are thus qualitatively consistent with an hypothesis of myogenically induced flow heterogeneity. However, data of Meininger et al. (1987) indicate that a regional difference of nearly 20 mm Hg in feed arteriolar pressure would be required to achieve a 42% disparity in blood flow via myogenic control of precapillary tone. Thus, even though significant flow reductions occur following pressure increases as small as 10 mm Hg (Meininger et al., 1987), it is clear from the present data that spatial variations in hamster cremaster cell flux are not simply the result of myogenic autoregulation. The potential consequences of blood flow heterogeneity are of sufficient concern that continued exploration of the mechanism involved is warranted. It appears likely that regional differences in blood flow distribution are caused by a combination of factors that may modulate one another. For example, intravascular pressure has been shown to alter the sensitivity of vessels to vasoconstrictor agonists (Tallarida et al., 1974; Lombard et al., 1990). Thus the regional response to circulating agonists may differ as a result of spatial variations in transmural pressure. Second, oxygen levels and the concentrations of cellular metabolites have been shown to modulate neural and local regulators of arteriolar tone (Marshall, 1982; Boegehold and Johnson, 1988; McGillivray-Anderson and Faber, 1990). Finally, it has been shown recently that EDRF inhibits myogenic tone
BRIEF
COMMUNICATION
117
(Griffith and Edwards, 1990). It is hoped that further experimentation will sort out the consequences of these complex interactions on blood flow heterogeneity. ACKNOWLEDGMENTS The authors thank Mr. John S. Rozum for his excellent technical assistance. This work was supported by Grants HL13437, HL07506, and HL07249 from the National Heart, Lung and Blood Institute. Preliminary results of this work were published previously in abstract form (FASEB. J. (1989) 3, A1402).
REFERENCES AUSTIN, R. E., ALDEA, G. S., COGGINS,D. L., FLYNN, A. E., AND HOFFMAN, J. I. E. (1990). Profound
spatial heterogeneity of coronary reserve: Discordance between patterns of resting and maximal myocardial blood flow. Circ. Res. 67, 319-331. BOEGEHOLD, M. A., AND JOHNSON, P. C. (1988). Periarteriolar and tissue PO2 during sympathetic escape in skeletal muscle. Am. J. Physiof. 254, H929-H936. BREZIS, M., ROSEN, S., SILVA, P., AND EPSTEIN, F. H. (1984). Renal ischemia: A new perspective. Kidney Int. 26, 375-383. DAVIS, M. J., FERRER, P. N., AND GORE, R. W. (1986). Vascular anatomy and hydrostatic pressure in the hamster cheek pouch. Am. J. Physiol. 250, H291-H303. DULING, B. R., AND BERNE, R. M. (1970). Longitudinal gradients in periarteriolar oxygen tension: A possible mechanism for the participation of oxygen in local regulation of blood flow. Circ. Res.
27, 669-678. GRIFFITH, T. M., AND EDWARDS, D. H. (1990). Myogenic autoregulation of flow may be inversely related to endothelium-derived relaxing factor activity. Am. J. Physiol. 258, H1171-H1180. HARGREAVES,D., EGGINTON, S., AND HUDLICK~~, 0. (1990). Changes in capillary perfusion induced by different patterns of activity in rat skeletal muscle. Microvasc. Res. 40, 14-28. LOMBARD, J. H., ESKINDER, H., KAUSER, K., OSBORN, J. L., AND HARDER, D. R. (1990). Enhanced norepinephrine sensitivity in renal arteries at elevated transmural pressure. Am. J. Physiol. 259,
H29-H33. MARSHALL, J. M. (1982). The influence of the sympathetic nervous system on individual vessels of the microcirculation of skeletal muscle of the rat. J. Physiol. 332, 169-186. MCGILLIVRAY-ANDERSON, K. M., AND FABER, J. E. (1990). Effect of acidosis on contraction of mi-
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