Ronald L. Dundore

Zaprinast increases cyclic GMP levels in plasma and in aortic tissue of rats

The purpose of this study was to determine if significant relationships exist between plasma and aortic cyclic GMP (cGMP) levels and pharmacodynamic effect after the i.v. administration of the cGMP-selective phosphodiesterase inhibitor zaprinast to conscious, spontaneously hypertensive rats. Zaprinast dose-dependently increased plasma and aortic cGMP levels at 10, 18 and 30 mg/kg and decreased mean arterial blood pressure (MAP) at 18 and 30 mg/kg. The concentrations of cGMP in the plasma and in the aorta were significantly correlated (r = 0.765, P < 0.0001). The changes in MAP were significantly correlated to aortic (r = -0.750, P < 0.0001) and plasma (r = -0.762, P < 0.0001) cGMP levels. We conclude that plasma cGMP may be an index of cGMP-selective phosphodiesterase inhibition in vivo.

Protein kinase inhibitors and blood pressure control in spontaneously hypertensive rats.

Considerable evidence suggests that protein kinase C activation participates in the regulation of vascular smooth muscle tone. The objective of the current study was to examine the relations between inhibition of protein kinase C (PKC) and myosin light-chain kinase (MLCK) and vasorelaxation and blood pressure regulation in spontaneously hypertensive rats (SHR). Putative PKC inhibitors from two chemical classes, staurosporinelike (staurosporine and K252A) and isoquinolinesulfonamides (H7 and HA1004), were tested for their ability to 1) inhibit PKC and MLCK from SHR aorta, 2) relax isolated SHR aorta, and 3) lower blood pressure in conscious SHR. A rank order of potency for the inhibition of PKC and MLCK was established, with the staurosporinelike compounds (staurosporine PKC IC50 = 54 nM) clearly more potent than the isoquinolinesulfonamides (H7 PKC IC50 = 128 microM). The rank order of potency for inhibition of PKC was retained for inhibition of MLCK for all compounds. Staurosporine (EC50 = 75 nM) and H7 (EC50 = 2 microM) caused concentration-dependent relaxation of SHR aorta, but only staurosporine produced vasorelaxation at concentrations consistent with the inhibition of PKC or MLCK. Dose-dependent reductions in arterial pressure of SHR were demonstrated after intravenous injection of staurosporine and HA1004. A single intravenous injection of staurosporine (0.3 mg/kg) lowered blood pressure for more than 10 hours. Staurosporine also lowered blood pressure after oral administration. The depressor response to staurosporine was unaffected by sympathetic beta-adrenergic blockade. In conclusion, the vasorelaxant and antihypertensive actions of staurosporine in SHR are consistent with the inhibition of PKC but could also be equally related to inhibition of MLCK. Not all PKC inhibitors produce vasorelaxation and lower blood pressure. Moreover, the lack of correlation between in vitro vasodilation and PKC or MLCK inhibition for the isoquinolinesulfonamide protein kinase inhibitors H7 and HA1004 suggests that these agents do not cause vasorelaxation in SHR by inhibition of these enzymes.

Differential hemodynamic responses to selective inhibitors of cyclic nucleotide phosphodiesterases in conscious rats.

Summary: Selective inhibition of either the low Km cyclic AMP (cAMP) or low Km cyclic GMP (cGMP) phosphodiesterase (PDE) promotes vasorelaxation and, consequently, produces depressor effects. To evaluate the systemic and regional hemodynamic effects of selective inhibitors of these PDE isozymes, CI-930 (0.1–10 mg/kg), an inhibitor of low Km cAMP PDE, or zaprinast (3–30 mg/kg), an inhibitor of low Km cGMP PDE, was given i.v. to conscious, normotensive rats. The rats were chronically instrumented with vascular catheters and either an ultrasonic transit-time flow probe around the ascending aorta or miniaturized pulsed Doppler flow probes around the superior mesenteric and left renal arteries and the abdominal aorta. CI-930 and zaprinast, at cumulative doses of 3 and 30 mg/kg, respectively, produced comparable reductions in mean arterial pressure (- 22 ± 3 and - 19 ± 4 mm Hg, respectively) and total peripheral resistance (-0.41 ± 0.07 and −0.42 ± 0.06 mm Hg/ml/min, respectively) but affected other hemodynamic variables differently. CI-930 at 3 mg/kg increased the heart rate (HR), maximal aortic flow acceleration (dF/dt), and peak aortic flow and decreased the stroke volume (SV). Cardiac output (CO) was not affected by CI-930. Zaprinast at 30 mg/kg increased the CO, dF/dt, and peak aortic blood flow. The HR and SV were unaffected by zaprinast. Although both CI-930 and zaprinast increased the dF/dt and peak aortic flow, these parameters were affected more by CI-930 than by zaprinast. CI-930 decreased hindquarter, mesenteric, and renal vascular resistances in a dose-dependent manner. Since the doses of zaprinast at which significant reductions in hindquarter, mesenteric, and renal vascular resistances first occurred were 10,18, and 30 mg/kg, respectively, these vascular beds may be differentially sensitive to the vasorelaxant effects of zaprinast. It is concluded that, although selective inhibitors of either low Km cGMP or low Km cAMP PDE decrease blood pressure by lowering the total peripheral resistance in conscious rats, the accompanying hemodynamic patterns for each agent are different.

Inhibition of low Km cyclic GMP phosphodiesterases and potentiation of guanylate cyclase activators by cicletanine.

Cicletanine is an antihypertensive/vasorelaxant/natriuretic agent of unknown mechanism. We wished (a) to determine if cicletanine interacts with guanylate cyclase activators that modulate vasomotor tone and sodium balance [i.e., atriopeptin II (AP II), endothelium-derived relaxing factor (EDRF), and sodium nitroprusside (SNP)], and (b) to define the subcellular basis for this interaction by quantitating the effects of cicletanine on low Km cyclic GMP phosphodiesterase (PDE) activity. In phenylephrine-contracted rat aortic smooth muscle, the vasorelaxant potency of cicletanine was increased twofold in the presence of a threshold-relaxant concentration of AP II, and functional cyclic GMP PDE inhibition was also evident from the three- to sixfold potentiation by cicletanine of AP II- or SNP-induced vasorelaxation. Vasorelaxation produced by cicletanine was not endothelium dependent, however. In further studies, intravenous (i.v.) administration of cicletanine or the low Km cyclic GMP PDE inhibitor, zaprinast, decreased blood pressure (BP) ≤20% in conscious spontaneously hypertensive rats (SHR). These results are consistent with the additional finding that cicletanine inhibited Ca2+ -calmodulin (CaM) cyclic GMP PDE and zaprinast-sensitive cyclic GMP specific PDE over a concentration range (10–600 μM) similar to that for vasorelaxation. Thus, inhibition of low Km cyclic GMP PDEs by cicletanine may be partly responsible for the vasorelaxant effect of cicletanine as well as the potentiation by cicletanine of the vasorelaxant actions of guanylate cyclase activators. The extent to which this mechanism contributes to the antihypertensive efficacy of cicletanine has not yet been fully determined.

Sodium nitroprusside potentiates the depressor response to the phosphodiesterase inhibitor zaprinast in rats

To determine if the presence of an activator of guanylate cyclase alters the depressor response to a selective inhibitor of low Km cyclic GMP (cGMP) phosphodiesterase (PDE), zaprinast (3-30 mg/kg) was given i.v. to conscious, spontaneously hypertensive rats during a steady state of i.v. infusion of sodium nitroprusside (15 micrograms/kg per min). Sodium nitroprusside significantly increased the magnitude of the depressor response to zaprinast. In contrast, fenoldopam (20 micrograms/kg per min), an activator of adenylate cyclase, did not affect the depressor response to zaprinast. Zaprinast (10 mg/kg) significantly decreased mean arterial pressure (MAP) in rats given an infusion of sodium nitroprusside, an activator of soluble guanylate cyclase, at doses of 15 and 25 micrograms/kg per min but not at a dose of 5 micrograms/kg per min. However, in rats given atrial natriuretic peptide (ANP; 0.5, 1 and 2 micrograms/kg per min), an activator of particulate guanylate cyclase, zaprinast (10 mg/kg) did not affect MAP. In contrast to the potentiation of the depressor response to zaprinast, sodium nitroprusside (15 micrograms/kg per min) significantly attenuated the reductions in MAP produced by CI-930, a selective inhibitor of low Km cAMP PDE. It is concluded that sodium nitroprusside, but not ANP or fenoldopam, potentiates the depressor response to zaprinast. Furthermore, the potentiation of the depressor response to zaprinast is dependent upon the dose of sodium nitroprusside and is selective for zaprinast; the depressor response to CI-930 is attenuated by sodium nitroprusside.

Zaprinast, but not dipyridamole, reverses hemodynamic tolerance to nitroglycerin in vivo

Abstract Hemodynamic tolerance to nitroglycerin was developed in spontaneously hypertensive rats following 2–3 days of pretreatment with 100 mg/kg of nitroglycerin administered s.c. 3 times/day. Tolerance was evaluated both in vivo, by administering ascending bolus doses of nitroglycerin of 1–300 μg/kg i.v., and ex vivo in isolated, denuded aortic vascular rings by exposure to ascending concentrations of nitroglycerin of 0.0003–100 μM. Tolerance was observed as a significant blunting of the hypotensive and vasorelaxant effect of nitroglycerin. Co-incubation of tolerant aortic rings and pretreatment of tolerant SHR with 10 μM and 0.1–10 mg/kg zaprinast, respectively, resulted in full restoration of the vasorelaxant and hypotensive effect of nitroglycerin. Zaprinast partially reversed hemodynamic tolerance at 0.01 mg/kg. Conversely, dipyridamole (10 μM) reversed tolerance ex vivo, but was ineffective in reversing tolerance in vivo at pretreatment doses of 30 and 60 mg/kg. Following a 100-μg/kg i.v. challenge dose of nitroglycerin, aortic cyclic guanosine monophosphate (cGMP) levels were lower in nitroglycerin tolerant SHR when compared to non-tolerant SHR. Pretreatment of tolerant SHR with 10 mg/kg zaprinast restored the increase in cGMP levels to nitroglycerin to that seen in non-tolerant SHR. Conversely, dipyridamole (30 mg/kg) pretreatment was not effective in restoring cGMP levels. These data therefore suggest that reversal of hemodynamic tolerance in vivo is related to restoration of changes in vascular cGMP levels. Zaprinast, a selective cGMP phosphodiesterase inhibitor, effectively reverses tolerance and dipyridamole, a rather non-selective inhibitor, does not.

Nω-Nitro-L-arginine attenuates the accumulation of aortic cyclic GMP and the hypotension produced by zaprinast

To determine if N omega-nitro-L-arginine (NNA), an inhibitor of the synthesis and/or release of endothelium-derived relaxing factor (EDRF), alters the response to zaprinast, a selective inhibitor of cyclic GMP (cGMP) phosphodiesterase, zaprinast (3-30 mg/kg) or vehicle (1 ml/kg) was given to conscious, spontaneously hypertensive rats (SHR) in a cumulative i.v. dose-response manner 30 min after pretreatment with NNA (1 or 3 mg/kg) or saline (1 ml/kg). Mean arterial pressure (MAP) was measured 5 min after each dose of zaprinast. Five minutes after the last dose of zaprinast (30 mg/kg), the rats were anesthetized with pentobarbital (25 mg i.v.). A segment of the abdominal aorta was freeze-clamped in situ and removed for the determination of cGMP levels. NNA (3 mg/kg) decreased basal aortic cGMP levels by 54% and increased MAP by 37 +/- 2 mm Hg. Zaprinast (30 mg/kg) increased aortic cGMP by 187% and decreased MAP by 49 +/- 4 mm Hg. NNA (3 mg/kg) reduced the accumulation of cGMP in aortic tissue (from 4.1 +/- 0.4 to 1.3 +/- 0.1 fmol/microgram protein) and attenuated the depressor response (from -49 +/- 4 to -31 +/- 4 mm Hg) produced by zaprinast. These data are consistent with the hypothesis that NNA inhibits the tonic release of EDRF and that the depressor effects of zaprinast are due, at least in part, to the potentiation of the vasodilator effects of EDRF in vivo. Moreover, since the changes in MAP produced by NNA and zaprinast were significantly correlated with cGMP levels in aortic tissue, the concentration of cGMP in vascular tissue may be a determinant of blood pressure in SHR.

Cardiovascular activity of WIN 65579, a novel inhibitor of cyclic GMP phosphodiesterase 5.

This study describes the phosphodiesterase inhibitory potency and cardiovascular actions of WIN 65579 (1-cyclopentyl-3-ethyl-6-(3-ethoxy-4-pyrridyl)-1H-pyrazolo[3,4-d]p yrimidin-4-one), a potent, new cGMP phosphodiesterase 5 inhibitor. WIN 65579 is a competitive inhibitor of phosphodiesterase 5, with IC50 values of 2-3 nM for phosphodiesterase 5 from human or canine vascular sources. WIN 65579 has low affinity for phosphodiesterases 1, 2 and 3 (IC50 > 3-10 microM), and is somewhat selective for phosphodiesterase 4 (IC50 approximately 100 nM). WIN 65579 is an endothelial-dependent relaxant of rat aortic smooth muscle (EC50 = 60 nM) and lowers mean arterial blood pressure in conscious spontaneous hypertensive rats following intravenous or oral dosing. WIN 65579 also increases plasma cGMP levels, and reinstates vascular responsiveness to nitroglycerin in conscious rats that are nitroglycerin-tolerant. These data show that WIN 65579 is one of the more potent phosphodiesterase 5 inhibitors, and that WIN 65579 possesses cardiovascular activities consistent with vascular phosphodiesterase 5 inhibition in vivo.

Hemodynamic and renal effects of the protein kinase inhibitor staurosporine in conscious rats.

Summary: To evaluate the pattern of hemodynamic responses produced by an inhibitor of protein kinase C (PKC), staurosporine 0.03–0.55 mg/kg was administered intravenously (i.v.) to conscious, normotensive rats chronically instrumented with vascular catheters for direct measurement of blood pressure (BP) and i.v. administration of drugs and either an aortic flow probe for measurement of cardiac output (CO) or miniaturized pulsed Doppler flow probes for measurement of hindquarter, renal, and mesenteric vascular resistances. Staurosporine decreased mean arterial pressure (MAP) and total peripheral resistance (TPR) and increased heart rate (HR) in a dose-dependent manner. Because staurosporine decreased resistance in all three vascular beds monitored (hindquarter, renal, and mesenteric), staurosporine is probably a nonselective vasodilator that decreases MAP by decreasing resistance in a number of peripheral vascular beds. Staurosporine produced biphasic effects on CO, dF/dtmaX and peak aortic blood flow; these parameters were significantly increased at doses <0.3 mg/kg and decreased to levels equal to or significantly less than control values at doses >0.3 mg/kg. In comparison, the calcium channel blocker nitrendipine decreased MAP and TPR and increased HR, CO, dF/dtmax, and peak aortic flow in a dose-dependent manner over the entire dose range (0.01–1 mg/kg i.v.). Staurosporine (0.3 mg/kg) and nitrendipine (1 mg/kg) produced similar changes in MAP (-44 ± 3 and −33 ± 2 mm Hg, respectively), yet staurosporine affected dFldtMAX to a lesser extent than nitrendipine (-5 ± 36 and 390 ± 46 ml/s/s, respectively). These data may indicate that the reflex increase in contractility produced by lower doses of staurosporine is limited by a negative inotropic effect of staurosporine which predominates at higher doses. The renal effects of staurosporine were assessed in conscious, hydrated rats. Staurosporine (0.1 mg/kg) significantly decreased urinary potassium excretion and increased the ratio of sodium to potassium, thereby promoting selective excretion of sodium. Staurosporine (0.3 mg/kg) had significant antinatriuretic and antidiuretic effects, however. We conclude that although staurosporine produced a pattern of hemodynamic and renal effects that may be desirable for new antihypertensive agents, it may be cardiotoxic at higher doses. Because staurosporine inhibits other protein kinases, a definitive evaluation of PKC inhibition as a potential method for treating hypertension requires development of more specific PKC inhibitors.

Pharmacologic and Pharmacodynamic Effects of the Selective Low Km Cyclic AMP Phosphodiesterase III Inhibitors WIN 63291 and WIN 62582

Summary We describe the biochemical, pharmacologic, and in vivo pharmacodynamic profiles of two novel inhibitors of the cyclic GMP-inhibitable, low Km cyclic AMP phosphodiesterase (PDE) III; WIN 63291, a 6-qui-nolinyl analogue of the prototypic PDE III inhibitor milrinone and WIN 62582, an imidazopyridinone. Both WIN 62582 and WIN 63291 competitively inhibit PDE HI from rat, dog, and human heart and from rat and canine aorta with IC50 values of 5–37 and 55–80 nM, respectively; the IC50 values for milrinone ranged from 300 to 520 nM. WIN 62582 and WIN 63291 are at least 1,000-fold selective for PDE III relative to inhibition of PDE isozymes I, II, IV, and V. We evaluated WIN 62582 and WIN 63291 in conscious rats and dogs after intravenous (i.v.) and oral (p.o.) administration. The dose of WIN 62582 required to reduce mean arterial blood pressure (MAP) by 20% (ED20) in rats was 1.8 mg/kg, with a pharmacodynamic duration of action of ±2 h. In comparison, the estimated i.v. ED20 for WIN 63291 in rats was 0.4 mg/kg, with a pharmacodynamic duration of action >6 h. In conscious dogs, the i.v. doses of WIN 62582 and 63291 required to increase left ventricular (LV) dP/dtmax significantly were 0.1 and 0.01 mg/kg, respectively. In dogs, WIN 63291 0.1 mg/kg p.o. increased LVdP/dtmax by 86% in 30 min; LVdp/dtmax remained increased by 60% for at least 6 h. In comparison, WIN 62582, 0.3 mg/kg p.o., increased LVdP/dt by 56% in 30 min and remained increased by 40% at 6 h. These data suggest that WIN 62582 and WIN 63291 are potent, selective, orally active inhibitors of PDE III and have a relatively long duration of action in vivo. Although WIN 62582 is 10-fold more potent than WIN 63291 as an inhibitor of PDE III in vitro, WIN 63291 is a slightly more potent cardiovascular agent in vivo after i.v. and p.o. administration to conscious rats and dogs.