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Opposition of the vasopressin-induced vasoconstriction in the isolated perfused rat kidney by some prostaglandins
PROSTAGLANDINS
OPPOSITION OF THE VASOPRESSIN-INDUCED VASOCONSTRICTION IN THE ISOLATED PERFUSED RAT KIDNEY BY SOME PROSTAGLANDINS Cecil R. Pace-Asciak and Alan Rosenthal Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Canada MSGlX8 ABSTRACT PGE2 can vasoconstrict or vasodilate the isolated Krebs-perfused rat kidney depending on the tone of the renal vasculature. Thus, it is weakly constrictor (threshold S-IO ng bolus dose) in the perfused kidney whose perfusion pressure is 47 2 2 SD mmHg (n = 61, but becomes a vasodilator (threshold -10 pg) in the kidney whose perfusion pressure has been raised to 73 f 6 SD mmHg (n = 6) or 121 + 8 SD mmHg (n = 6) through constant infusion of Vasopressin (0.1 and 0.25 mu/ml respectively). PGE, was equally effective as PGE2 while other PGs, 12, I,, and 6-keto El, were less effective in opposing vasoconstriction. PGF2, was inactive up to a dose of 10 ng. INTRODUCTION The isolated Krebs-perfused kidney of the rat has been shown to respond differently to vasodepressor prostaglandins such as PGE and PGA2, than kidneys of other species (1). Thus while 6GE2 opposes vasoconstrictor stimuli in the isolated kidney of the rabbit, studies with the rat kidney have shown PGE2 to act as a vasoconstrictor (1). This suggested a rather unique pro-hypertensive behavior for PGE2 in the rat especially since ample evidence exists to show that systemically PGE2 is capable of lowering blood pressure (2-S). In this report we describe the effects of PGE2 and some other PGs on the renal vascular resistance of the isolated rat kidney at low vascular resistance and after resistance had been raised through constant infusion with two cencentrations of Vasopressin. METHODS Adult male Wistar rats (250-300 g) were purchased from a local supplier and maintained in our animal quarters with free access to rat chow (Purina) and tap water. Prostaglandin stock solutions were prepared in ethanol (100%) at a
OCTOBER
1981 VOL. 22 NO. 4
567
PROSTAGLANDINS
concentration of 1 ug/ul and aliquots from this stock were diluted with prefiltered Krebs-bicarbonate perfusing medium to a concentration range of 0.02-500 ng/ml. PG12 was dissolved in cold Krebs medium adjusted to pH 8.2 with sodium bicarbonate. Bolus 50 ul injections were made with a Hamilton syringe into the perfusing medium via a septum in the renal arterial cannula. Vasopressin solutions were made up in filtered Krebs perfusing medium. This was infused with a Harvard pump at a rate of 0.016 ml/min through a side arm connected to the renal arterial cannula and mixed with Krebs perfusing medium prior to entry into the kidney. The flow rate was maintained constant at 6-7 ml/min. The isolated
kidney preparation
The isolated kidney preparation was set up according to Nishiitsutsuji-Uwo et al. (6). After an abdominal incision, the right ureter was cannulated with PE-10 polyethylene tubing. The following vessels were ligated: the aorta below the superior mesenteric artery,the inferior vena cava below the left renal vein as well as the left renal vein. A perforated stainless steel cannula was inserted into the inferior vena cava between the left and right renal veins and advanced to the right renal vein to collect the renal venous effluent. Continuous renal perfusion with Krebs medium (4-5 ml/min initially) was carried out via a 20-guage needle inserted through the superior mesenteric artery and advanced into the right renal artery. The aorta was ligated above the superior mesenteric artery and the inferior vena cava was ligated below the liver. The kidney was excised from its surrounding tissue and placed horizontally on moist pads in a water jacketed (37') plastic box. The perfusing medium consisted of a Krebs-bicarbonate buffer containing 11 mM glucose, 2 U/L Insulin and an electrolyte composition of Na 141.1 mM, K 5.9 mM, Ca 1.25 mM, Mg 1.0 mM, Cl 122.15 mM, SO4 1.0 mM, PO 1.1 mM and HCO 25 mM. The solution was gassed with 954 0 - 5% CO an a pumped at constant flow through a glass hgating co?l, 2 filter traps and 2 bubble traps before entering the kidney. Because the effect of PGs on renal vascular resistance was investigated the perfusion system consisted of a single pass system, with the venous effluent being discarded. An equilibration period of 30-60 min was allowed before testing of drugs. Measurement
of perfusion
pressure
Constant flow to the kidney was maintained at 6-7 ml/ min with a Masterflex peristaltic pump. Perfusion pressure was measured with a miniature arterial pressure transducer (Statham P23DB) attached on line with the arterial cannula via a 3-way connecting tube. With flow maintained constant,
568
OCTOBER
1981 VOL. 22 NO. 4
PROSTAGLANDINS
any changes in resistance are manifested by a corresponding change in perfusion pressure which was continuously monitored on a Brush 2200 recorder. RESULTS During the initial set up conditions (perfusion pressure 47 ? 2 SD mmHg, n = 6) bolus intraarterial injections of PGE2 caused dose-related weak pressor responses (threshold 5-10 ng). When the renal resistance was raised through a constant infusion of Vasopressin, 0.1 mu/ml (perfusion pressure 73 i 6 SD mmHg, n = 6) or 0.25 mu/ml (perfusion pressure 121 f 8 SD nunHg, n = 6) PGE2 exerted potent doserelated vasodepressor responses with a threshold around 10 pg (Fig. 1). At high doses (above 20 ng) the response to PGE2 became biphasic showing a rapid vasoconstrictor phase
vp=o
........ 2.5
I
10
5
PGE2
40
20
160
80
(ngl
Vp=O.lO
KB
0.05
0.1
0.5
1
2.5
mu/ml
10
20
40
Fig. 1. Profiles showing changes in perfusion pressure (flow maintained constant, 6.5 ml/min) resulting from bolus intraarterial injections (50 1.11)of varying amounts of PGE2 (10 pg - 160 ng) in the absence (top) and presence (bottom) of Vasopressin (VP) in the perfusing medium. Note that while responses to PGE2 are only vasoconstrictor when Vp = 0, they are vasodilator at low doses of PGE when Vp = 0.10 mu/ml. At high doses of PGE2, $he vasoconstrictor component appears again (see arrow) i.e. responses become biphasic. KB = Krebs vehicle.
OCTOBER
1981 VOL. 22 NO. 4
569
PROSTAGLANDINS
followed hy a slow vasodilator phase whose amplitude was inversely related to the amount of PGE2 injected (Fig. 1) presumably due to an attenuation by the vasoconstrictor phase. At low doses (10-5000 pg) the principal response to PGE2 was vasodilation while the constrictor phase became apparent only at high doses (>lO ng) at both low and high renal vascular resistance. This is shown in the dose response relationships in Figure 2 at two cencentrations of Vasopressin (0.10 and 0.25 mu/ml). The vasodilation by PGE2 seems directly related to the concentration of Vasopressin in the perfusing medium and always occurs when Vasopressin is present in the medium; the greater the Vasopressin-induced constriction the greater the vasodilation by a given dose of PGE2 (see for example 500 pg dose of PGE2 when only
Fig. 2 (left). Log dose response biphasic effects of PGE2 on the perfusion pressure (flow kept constant) of the isolated rat kidney perfused with Krebs-bicarbonate buffer containing Vasopressin (0.1 mu/ml and 0.25 mu/ml). Values represent mean f SD, number of experiments in brackets. Changes in pressure are represented by (+) if effects are constrictor or (-) if effects are dilator. Top part of Figure represents the rapid vasoconstrictor component preceding the vasodilation (see Fig. 1 bottom for profile). Fig. 3 (right). Comparative vasodilator potency of various PGs expressed relative to PGE2 (%), on the perfusion pressure of the isolated perfused rat kidney. Data represents mean f SEM, number of experiments = 6.
OCTOBER 1981 VOL. 22 NO. 4
PROSTAGLANDINS
a dilator response is seen at both concentrations of Yasopressin). On the other hand, the rapid constrictor component of PGE2 is inversely related to the Yasopressin concentration and appears only at the higher doses of PGE2 in the presence or absence,of Vasopressin. Thus the higher the Vasopressin concentration the lower is the constriction by PGE2 presumably because the response to PGE2 at the higher amounts of Vasopressin is biphasic i.e. the rapid pressor phase is attenuated by a strong depzsor phase. Other PGs including two synthetic products, PGIl and 6-keto PGEl, were tested for their ability to oppose the Vasopressin-induced vasoconstriction of the isolated kidney. All PGs, except PGF c, produced slow vasodepressor responses P 8 F2o was inactive within the dose range similar to PGE2. tested. Each preparation (n = 6) was tested with all the above prostaglandins sequentially in increasing doses ranging from 10 pg to 25 ng; thus all the prostaglandins were analyzed at one dose before the next dose was investigated. Data from these analyses was expressed as percentage activity relative to PGE2 and is shown in Figure 3. PGEl and PGE2 were by far the most potent of the compounds investigated 6-keto-PGEl, PG12 and with a threshold dose around 10 pg. PGIl were over 5 fold less active than PGE2 especially at bolus doses lower than 500 pg while PGF20 even at a bolus dose of 2 ng was inactive. At this dose, PGE2 produced a 25 mmHg drop in pressure. At doses higher than 10 ng, a rapid pressor component similar to the response observed in the absence of Vasopressin, began to appear. Consequently we did not investigate doses of PGs higher than 25 ng. DISCUSSION and PGA2, Unlike the rabbit kidney which dilates to PGE een shown the isolated Krebs perfused kidney of the rat has l? to vasoconstrict to PGE2, PGF2, and PGA2 and to enhance rather than oppose the vasoconstriction due to sympathetic nerve stimulation (1). We have recently confirmed these findings in kidneys considered to be maximally dilated but we have also shown that PGE2 has biphasic effects on the renal vasculature which are dependent on the level of its is resistance (10). Thus at low vascular resistance, PGE weakly constrictor probably because the renal vascula zure is greatly dilated. In fact, nitroglycerin and nitroprusside are mostly inactive on this preparation (unpublished observations). However, when the rat renal vasculature is partly constricted with a constant infusion of Vasopressin, PGE2 becomes a potent vasodilator whose threshold lies around 10 pg (Figs. 1, 2). The ability to oppose the vasopressin-induced vasoconstriction of the isolated rat kidney is not restricted
OCTOBER
1981 VOL. 22 NO. 4
571
PROSTAGLANDINS
solely to PGE2. Other PGs including PGEl, PGI2, and the two chemically synthetic analogs PGIl and Ci-ketoPGEl also share in this property. Our results indicate that the renal vasculature of the rat is highly responsive to both PGE and PGE2 (approximately to the same extent - see Fig. 3L The threshold dose is well within the expected physiological concentration range for these products (around 10 pg). It is interesting to note that the presence of a 6-keto group such as in 6-keto PGEl alters the biological activity of the molecule considerably in this preparation decreasing its effect by about 5 fold. Similarly the presence of a 6(9)-oxy ring structure as in PGI or PGI instead of the 9-keto group of the PGEs also inf3uences &he biological potency of the compound decreasing it by about 5-10 fold over the respective PGE compound. Whether this effect is due to lack of permeability or uptake of the bicyclic products, PGI2 or PGI by the renal vasculature is not known at this time but tA' e similar biological potency of the 5,6dihydro product (PGI ) relative to PGI2 is worth noting and possibly is worthy o 4 further study. Reduction of the 9keto group in PGE as in PGF2c completely abolishes activity at least within tl? e range of concentrations tested in this study. We do not know whether the observations made in this study apply to the in vivo situation. PGE2 is abundantly formed in the rat kidng7-9) but whether it participates in the regulation of the tone of the renal vasculature is not known. Indirect studies using indomethacin have previously suggested that some vasodilator PGs might participate in renal hemodynamics (11). Whether such experiments, however, point to a participation of PGs only of renal origin and exclude extra renal PGs (i.e. aortic) cannot be determined, Our present experiments showing that PGI2 is about 5-10 fold less potent than PGE2 in opposing renal vasoconstriction are worthy of comment. We have recently shown that aortic tissue from spontaneously hypertensive rats is capable of forming PGI2 in greater amounts than PGE while normal aorta forms approximately equal amounts of P8E2 and PGI2 (5). Furthermore, aortic production of PGI2 increases with age (l-5 months) parallelling the increase in systemic arterial blood pressure in the spontaneously hypertensive rat (12). It would thus seem that the hypertensive vasculature "turns on" the PGI2 synthase as if to compensate for its enhanced susceptibility to vasoconstrictor hormones. However, the enhanced production of PG12 by the hypertensive vasculature appears to be a poor compensatory mechanism since PGI2 has far less vasodilator potency on the renal vasculature than PGE It would probably have been more effective if PGE2 insZ' ead of PGI ) production were stimulated either intra or extra renalzy since this is the most potent of the series
572
OCTOBER
1981 VOL. 22 NO. 4
PROSTAGLANDINS
of products found in our study. We have not investigated the PGD2 or 6-keto-PGF effects of thromboxane B vasculature as these pro2' ucts are not likely 6: ;: ::;yr:;? tive within the range of concentrations tested in our study. If the vasodilator effects for the "natural" PGs shown in this study apply -in vivo, the balance between the production of renal and extrarenal PGs and the concentration of pressor hormones in the circulation could probably be an important determinant of the level of tone of the renal and possibly other vascular beds. ACKNOWLEDGEMENTS Prostaglandins El, E2, F 6-keto-PGE and 68-5,6dihydro-PGI (PGIl) were prove 2ZY ed by Drs. Uao Axen and John Pike, 3he Upjohn Company, Kalamazoo. PGI2-Na salt was the generous gift of Dr. K.C. Nicolaou, University of Pennsylvania. This study was supported by funds to C.P.-A. from the Medical Research Council of Canada (MT-4181). REFERENCES 1.
Malik, K.U. and McGiff, J.C.: Modulation by prostaglandins of adrenergic transmission in the isolated perfused rabbit and rat kidney. Circ. Res. -36:599, 1975.
2.
Bergstrom, S., Carlson, L.A., Eklund, L.G. and Or%l,L.: Cardiovascular and metabolic response to infusions of prostaglandin El in man. Acta Physiol. Stand. -64~332, 1965.
3.
Malik, K.U. and McGiff, J.C.: Cardiovascular actions of prostaglandins. In: Prostaglandins: Physiological, Pharmacological and Pathological Aspects, Adv. in Prostaglandin Res. (S.M.M. Karim, ed.) MTP Press, England, 1976, p. 103.
4.
Armstrong, J.M., Blackwell, G.J., Flower, R.J., McGiff, J.C., Mullane, K.M. and Vane, J.R.: Genetic hypertension in rats is accompanied by a defect in renal prostaglandin catabolism. Nature 260: 582, 1976.
5.
Pace-Asciak, C.R., Carrara, M.C., Rangaraj, G. and Nicolaou, K.C.: Enhanced formation of PGI a potent hypotensive substance, by aortic rings an2'homogenates of the spontaneously hypertensive rat. Prostaglandins -15:1005, 1978.
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1981 VOL. 22 NO. 4
573
PROSTAGLANDINS
6.
Nishiitsutsuji-Uwo, J.M., Ross, B.D. and Webs, H.A.: Metabolic activities of the isolated perfused rat kidney. Biochem. J. 103:852, 1967.
7.
Lee, J.B., Crowshaw, X., Takman, B.H., Attrep, K.A. and Gougoutas, J.Z.: The identification of prostaglandins E2, F and A2 from rabbit kidney medulla. Biothem. J. -. 16Y-1251, 1967.
8.
Anggard, E., Bohman, S.O., Griffin III, J.E., Larsson, C. and Maunsbach, A.B.: Subcellular localisation of the prostaglandin system in the rabbit renal papilla. Acta Physiol. Stand. 84:231, 1972.
9.
Hamberg, M.: Biosynthesis of prostaglandins in the renal medulla of rabbit. FEBS Lett. 2:127, 1969.
10.
Rosenthal, A.R. and Pace-Asciak, C.R.: Prostaglandin E2 potently opposes the vasopressin-induced constriction in the isolated perfused rat kidney. Fed. Proc. -39:3832, 1980.
11.
Lonigro, A.J., Itskovitz, H.D., Crowshaw, K. and McGiff, J.C.: Dependency of renal blood flow on prostaglandin synthesis in the dog. Circ. Res. 32:712, 1973.
12.
Pace-Asciak, C.R. and Carrara, M.C.: Age-dependent increase in the formation of prostaglandin I2 by intact and homogenised aortae from the spontaneously hypertensive rat. Biochim. Biophys. Acta 574:177, 1978.
Editor: Alan Nies Received: 9-2-80 Accepted: 8-18-81
574
OCTOBER
1981VOL.22N0.4
OPPOSITION OF THE VASOPRESSIN-INDUCED VASOCONSTRICTION IN THE ISOLATED PERFUSED RAT KIDNEY BY SOME PROSTAGLANDINS Cecil R. Pace-Asciak and Alan Rosenthal Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Canada MSGlX8 ABSTRACT PGE2 can vasoconstrict or vasodilate the isolated Krebs-perfused rat kidney depending on the tone of the renal vasculature. Thus, it is weakly constrictor (threshold S-IO ng bolus dose) in the perfused kidney whose perfusion pressure is 47 2 2 SD mmHg (n = 61, but becomes a vasodilator (threshold -10 pg) in the kidney whose perfusion pressure has been raised to 73 f 6 SD mmHg (n = 6) or 121 + 8 SD mmHg (n = 6) through constant infusion of Vasopressin (0.1 and 0.25 mu/ml respectively). PGE, was equally effective as PGE2 while other PGs, 12, I,, and 6-keto El, were less effective in opposing vasoconstriction. PGF2, was inactive up to a dose of 10 ng. INTRODUCTION The isolated Krebs-perfused kidney of the rat has been shown to respond differently to vasodepressor prostaglandins such as PGE and PGA2, than kidneys of other species (1). Thus while 6GE2 opposes vasoconstrictor stimuli in the isolated kidney of the rabbit, studies with the rat kidney have shown PGE2 to act as a vasoconstrictor (1). This suggested a rather unique pro-hypertensive behavior for PGE2 in the rat especially since ample evidence exists to show that systemically PGE2 is capable of lowering blood pressure (2-S). In this report we describe the effects of PGE2 and some other PGs on the renal vascular resistance of the isolated rat kidney at low vascular resistance and after resistance had been raised through constant infusion with two cencentrations of Vasopressin. METHODS Adult male Wistar rats (250-300 g) were purchased from a local supplier and maintained in our animal quarters with free access to rat chow (Purina) and tap water. Prostaglandin stock solutions were prepared in ethanol (100%) at a
OCTOBER
1981 VOL. 22 NO. 4
567
PROSTAGLANDINS
concentration of 1 ug/ul and aliquots from this stock were diluted with prefiltered Krebs-bicarbonate perfusing medium to a concentration range of 0.02-500 ng/ml. PG12 was dissolved in cold Krebs medium adjusted to pH 8.2 with sodium bicarbonate. Bolus 50 ul injections were made with a Hamilton syringe into the perfusing medium via a septum in the renal arterial cannula. Vasopressin solutions were made up in filtered Krebs perfusing medium. This was infused with a Harvard pump at a rate of 0.016 ml/min through a side arm connected to the renal arterial cannula and mixed with Krebs perfusing medium prior to entry into the kidney. The flow rate was maintained constant at 6-7 ml/min. The isolated
kidney preparation
The isolated kidney preparation was set up according to Nishiitsutsuji-Uwo et al. (6). After an abdominal incision, the right ureter was cannulated with PE-10 polyethylene tubing. The following vessels were ligated: the aorta below the superior mesenteric artery,the inferior vena cava below the left renal vein as well as the left renal vein. A perforated stainless steel cannula was inserted into the inferior vena cava between the left and right renal veins and advanced to the right renal vein to collect the renal venous effluent. Continuous renal perfusion with Krebs medium (4-5 ml/min initially) was carried out via a 20-guage needle inserted through the superior mesenteric artery and advanced into the right renal artery. The aorta was ligated above the superior mesenteric artery and the inferior vena cava was ligated below the liver. The kidney was excised from its surrounding tissue and placed horizontally on moist pads in a water jacketed (37') plastic box. The perfusing medium consisted of a Krebs-bicarbonate buffer containing 11 mM glucose, 2 U/L Insulin and an electrolyte composition of Na 141.1 mM, K 5.9 mM, Ca 1.25 mM, Mg 1.0 mM, Cl 122.15 mM, SO4 1.0 mM, PO 1.1 mM and HCO 25 mM. The solution was gassed with 954 0 - 5% CO an a pumped at constant flow through a glass hgating co?l, 2 filter traps and 2 bubble traps before entering the kidney. Because the effect of PGs on renal vascular resistance was investigated the perfusion system consisted of a single pass system, with the venous effluent being discarded. An equilibration period of 30-60 min was allowed before testing of drugs. Measurement
of perfusion
pressure
Constant flow to the kidney was maintained at 6-7 ml/ min with a Masterflex peristaltic pump. Perfusion pressure was measured with a miniature arterial pressure transducer (Statham P23DB) attached on line with the arterial cannula via a 3-way connecting tube. With flow maintained constant,
568
OCTOBER
1981 VOL. 22 NO. 4
PROSTAGLANDINS
any changes in resistance are manifested by a corresponding change in perfusion pressure which was continuously monitored on a Brush 2200 recorder. RESULTS During the initial set up conditions (perfusion pressure 47 ? 2 SD mmHg, n = 6) bolus intraarterial injections of PGE2 caused dose-related weak pressor responses (threshold 5-10 ng). When the renal resistance was raised through a constant infusion of Vasopressin, 0.1 mu/ml (perfusion pressure 73 i 6 SD mmHg, n = 6) or 0.25 mu/ml (perfusion pressure 121 f 8 SD nunHg, n = 6) PGE2 exerted potent doserelated vasodepressor responses with a threshold around 10 pg (Fig. 1). At high doses (above 20 ng) the response to PGE2 became biphasic showing a rapid vasoconstrictor phase
vp=o
........ 2.5
I
10
5
PGE2
40
20
160
80
(ngl
Vp=O.lO
KB
0.05
0.1
0.5
1
2.5
mu/ml
10
20
40
Fig. 1. Profiles showing changes in perfusion pressure (flow maintained constant, 6.5 ml/min) resulting from bolus intraarterial injections (50 1.11)of varying amounts of PGE2 (10 pg - 160 ng) in the absence (top) and presence (bottom) of Vasopressin (VP) in the perfusing medium. Note that while responses to PGE2 are only vasoconstrictor when Vp = 0, they are vasodilator at low doses of PGE when Vp = 0.10 mu/ml. At high doses of PGE2, $he vasoconstrictor component appears again (see arrow) i.e. responses become biphasic. KB = Krebs vehicle.
OCTOBER
1981 VOL. 22 NO. 4
569
PROSTAGLANDINS
followed hy a slow vasodilator phase whose amplitude was inversely related to the amount of PGE2 injected (Fig. 1) presumably due to an attenuation by the vasoconstrictor phase. At low doses (10-5000 pg) the principal response to PGE2 was vasodilation while the constrictor phase became apparent only at high doses (>lO ng) at both low and high renal vascular resistance. This is shown in the dose response relationships in Figure 2 at two cencentrations of Vasopressin (0.10 and 0.25 mu/ml). The vasodilation by PGE2 seems directly related to the concentration of Vasopressin in the perfusing medium and always occurs when Vasopressin is present in the medium; the greater the Vasopressin-induced constriction the greater the vasodilation by a given dose of PGE2 (see for example 500 pg dose of PGE2 when only
Fig. 2 (left). Log dose response biphasic effects of PGE2 on the perfusion pressure (flow kept constant) of the isolated rat kidney perfused with Krebs-bicarbonate buffer containing Vasopressin (0.1 mu/ml and 0.25 mu/ml). Values represent mean f SD, number of experiments in brackets. Changes in pressure are represented by (+) if effects are constrictor or (-) if effects are dilator. Top part of Figure represents the rapid vasoconstrictor component preceding the vasodilation (see Fig. 1 bottom for profile). Fig. 3 (right). Comparative vasodilator potency of various PGs expressed relative to PGE2 (%), on the perfusion pressure of the isolated perfused rat kidney. Data represents mean f SEM, number of experiments = 6.
OCTOBER 1981 VOL. 22 NO. 4
PROSTAGLANDINS
a dilator response is seen at both concentrations of Yasopressin). On the other hand, the rapid constrictor component of PGE2 is inversely related to the Yasopressin concentration and appears only at the higher doses of PGE2 in the presence or absence,of Vasopressin. Thus the higher the Vasopressin concentration the lower is the constriction by PGE2 presumably because the response to PGE2 at the higher amounts of Vasopressin is biphasic i.e. the rapid pressor phase is attenuated by a strong depzsor phase. Other PGs including two synthetic products, PGIl and 6-keto PGEl, were tested for their ability to oppose the Vasopressin-induced vasoconstriction of the isolated kidney. All PGs, except PGF c, produced slow vasodepressor responses P 8 F2o was inactive within the dose range similar to PGE2. tested. Each preparation (n = 6) was tested with all the above prostaglandins sequentially in increasing doses ranging from 10 pg to 25 ng; thus all the prostaglandins were analyzed at one dose before the next dose was investigated. Data from these analyses was expressed as percentage activity relative to PGE2 and is shown in Figure 3. PGEl and PGE2 were by far the most potent of the compounds investigated 6-keto-PGEl, PG12 and with a threshold dose around 10 pg. PGIl were over 5 fold less active than PGE2 especially at bolus doses lower than 500 pg while PGF20 even at a bolus dose of 2 ng was inactive. At this dose, PGE2 produced a 25 mmHg drop in pressure. At doses higher than 10 ng, a rapid pressor component similar to the response observed in the absence of Vasopressin, began to appear. Consequently we did not investigate doses of PGs higher than 25 ng. DISCUSSION and PGA2, Unlike the rabbit kidney which dilates to PGE een shown the isolated Krebs perfused kidney of the rat has l? to vasoconstrict to PGE2, PGF2, and PGA2 and to enhance rather than oppose the vasoconstriction due to sympathetic nerve stimulation (1). We have recently confirmed these findings in kidneys considered to be maximally dilated but we have also shown that PGE2 has biphasic effects on the renal vasculature which are dependent on the level of its is resistance (10). Thus at low vascular resistance, PGE weakly constrictor probably because the renal vascula zure is greatly dilated. In fact, nitroglycerin and nitroprusside are mostly inactive on this preparation (unpublished observations). However, when the rat renal vasculature is partly constricted with a constant infusion of Vasopressin, PGE2 becomes a potent vasodilator whose threshold lies around 10 pg (Figs. 1, 2). The ability to oppose the vasopressin-induced vasoconstriction of the isolated rat kidney is not restricted
OCTOBER
1981 VOL. 22 NO. 4
571
PROSTAGLANDINS
solely to PGE2. Other PGs including PGEl, PGI2, and the two chemically synthetic analogs PGIl and Ci-ketoPGEl also share in this property. Our results indicate that the renal vasculature of the rat is highly responsive to both PGE and PGE2 (approximately to the same extent - see Fig. 3L The threshold dose is well within the expected physiological concentration range for these products (around 10 pg). It is interesting to note that the presence of a 6-keto group such as in 6-keto PGEl alters the biological activity of the molecule considerably in this preparation decreasing its effect by about 5 fold. Similarly the presence of a 6(9)-oxy ring structure as in PGI or PGI instead of the 9-keto group of the PGEs also inf3uences &he biological potency of the compound decreasing it by about 5-10 fold over the respective PGE compound. Whether this effect is due to lack of permeability or uptake of the bicyclic products, PGI2 or PGI by the renal vasculature is not known at this time but tA' e similar biological potency of the 5,6dihydro product (PGI ) relative to PGI2 is worth noting and possibly is worthy o 4 further study. Reduction of the 9keto group in PGE as in PGF2c completely abolishes activity at least within tl? e range of concentrations tested in this study. We do not know whether the observations made in this study apply to the in vivo situation. PGE2 is abundantly formed in the rat kidng7-9) but whether it participates in the regulation of the tone of the renal vasculature is not known. Indirect studies using indomethacin have previously suggested that some vasodilator PGs might participate in renal hemodynamics (11). Whether such experiments, however, point to a participation of PGs only of renal origin and exclude extra renal PGs (i.e. aortic) cannot be determined, Our present experiments showing that PGI2 is about 5-10 fold less potent than PGE2 in opposing renal vasoconstriction are worthy of comment. We have recently shown that aortic tissue from spontaneously hypertensive rats is capable of forming PGI2 in greater amounts than PGE while normal aorta forms approximately equal amounts of P8E2 and PGI2 (5). Furthermore, aortic production of PGI2 increases with age (l-5 months) parallelling the increase in systemic arterial blood pressure in the spontaneously hypertensive rat (12). It would thus seem that the hypertensive vasculature "turns on" the PGI2 synthase as if to compensate for its enhanced susceptibility to vasoconstrictor hormones. However, the enhanced production of PG12 by the hypertensive vasculature appears to be a poor compensatory mechanism since PGI2 has far less vasodilator potency on the renal vasculature than PGE It would probably have been more effective if PGE2 insZ' ead of PGI ) production were stimulated either intra or extra renalzy since this is the most potent of the series
572
OCTOBER
1981 VOL. 22 NO. 4
PROSTAGLANDINS
of products found in our study. We have not investigated the PGD2 or 6-keto-PGF effects of thromboxane B vasculature as these pro2' ucts are not likely 6: ;: ::;yr:;? tive within the range of concentrations tested in our study. If the vasodilator effects for the "natural" PGs shown in this study apply -in vivo, the balance between the production of renal and extrarenal PGs and the concentration of pressor hormones in the circulation could probably be an important determinant of the level of tone of the renal and possibly other vascular beds. ACKNOWLEDGEMENTS Prostaglandins El, E2, F 6-keto-PGE and 68-5,6dihydro-PGI (PGIl) were prove 2ZY ed by Drs. Uao Axen and John Pike, 3he Upjohn Company, Kalamazoo. PGI2-Na salt was the generous gift of Dr. K.C. Nicolaou, University of Pennsylvania. This study was supported by funds to C.P.-A. from the Medical Research Council of Canada (MT-4181). REFERENCES 1.
Malik, K.U. and McGiff, J.C.: Modulation by prostaglandins of adrenergic transmission in the isolated perfused rabbit and rat kidney. Circ. Res. -36:599, 1975.
2.
Bergstrom, S., Carlson, L.A., Eklund, L.G. and Or%l,L.: Cardiovascular and metabolic response to infusions of prostaglandin El in man. Acta Physiol. Stand. -64~332, 1965.
3.
Malik, K.U. and McGiff, J.C.: Cardiovascular actions of prostaglandins. In: Prostaglandins: Physiological, Pharmacological and Pathological Aspects, Adv. in Prostaglandin Res. (S.M.M. Karim, ed.) MTP Press, England, 1976, p. 103.
4.
Armstrong, J.M., Blackwell, G.J., Flower, R.J., McGiff, J.C., Mullane, K.M. and Vane, J.R.: Genetic hypertension in rats is accompanied by a defect in renal prostaglandin catabolism. Nature 260: 582, 1976.
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Editor: Alan Nies Received: 9-2-80 Accepted: 8-18-81
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