T. Tanikella

VPA-985, a nonpeptide orally active and selective vasopressin V2 receptor antagonist.

The introduction of the thiazides as orally-active diuretics about forty years ago (1), and other more effective low-ceiling diuretics thereafter, revolutionized the treatment of edema, ascites, hypertension and related diseases. Later, the addition of potent high-ceiling (loop) and potassium-sparing diuretics provided clinicians with a wide choice of diuretics to eliminate retained sodium and water (2). However, it was soon evident that many patients became refractory to these saluretic agents and some developed hyponatremia (serum sodium < 130 mEq/L) (3, 4, 5). Hyponatremia also occurs in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), in patients with congestive heart failure (CHF), liver cirrhosis with ascites, renal failure, and many other disorders where the plasma vasopressin concentrations are inappropriately high for any given plasma osmolality. Under these conditions, an aquaretic (water diuretic), not a conventional diuretic, would be the drug of choice to promote the excretion of the retained body water and to normalize plasma osmolality and sodium concentration (6, 7, 8). As vasopressin (AVP, antidiuretic hormone (ADH)) acting at V2 receptors in the collecting ducts controls water re-absorption (7, 8), considerable effort has been spent over many years to develop vasopressin Vz receptor antagonists or agents that could inhibit the release of vasopressin from the posterior pituitary (8,9). Many peptide vasopressin analogs have been developed as vasopressin V2 receptor antagonists, and two of them, SK&F 101926 and SK&F 105494, showed excellent V2 antagonistic activity in many animal models, including nonhuman primates. Unfortunately, in humans, both compounds behaved as vasopressin V2 agonists (9). Recently, three nonpeptidic and orally active vasopressin receptor antagonists have been described in the literature. The first two, OPC-31260 (10), and SR 121463A (11), are V2 selective, while the third compound, YM087 (12), is a dual V1a/V2 receptor antagonist.

Endothelium-dependent basilar and aortic vascular responses in normotensive and coarctation hypertensive rats.

The responsiveness of acetylcholine (ACh), nitroglycerin (NG) and norepinephrine (NE) (aorta only) in both basilar arteries (BA) and thoracic aortic (TA) rings from coarctation hypertensive rats (CHR) were studied and compared to their sham-operated normotensive control rats (SNR). The effects of these agents were also evaluated in TA or BA with and without endothelium from naive normotensive rats (NNR). Blood pressure (BP) and plasma renin activity (PRA) of CHR were significantly higher than their time-matched SNR. Endothelium removal from TA of NNR significantly enhanced NE and NG sensitivity and reduced the maximum ACh relaxation. Removal of BA endothelium of NNR abolished ACh-induced relaxation but had no effect on NG-induced relaxation. In BA from CHR at any stage of hypertension studied, the sensitivity and maximum relaxation induced by ACh or NG were not significantly different than their respective time-matched SNR. ACh sensitivity of TA did not change in 1 Day CHR but decreased in 4 and 14 Day CHR. NG sensitivity increased, did not change and decreased in 1, 4 and 14 Day CHR, respectively. NE sensitivity increased in all stages of hypertension. These data suggest that in coarctation-induced hypertension there is a complex progression of events in TA which is modulated by different mechanisms as evidenced by the changes in the effects of NE, ACh and NG at various stages of hypertension. The results also suggest that the vascular endothelium of TA but not of BA may provide an acute protective mechanism to counteract the imbalance created by the increased sensitivity of smooth muscle cells to contractile agonists in the early stage of hypertension. However, persistent hypertension appears to override this mechanism.

Effect of the time interval between blood sampling and assay on serum angiotensin I-converting enzyme activity from captopril-treated rats

Intra-arterial injection of captopril (1 mg/kg) effectively lowered arterial blood pressure in aorta-coarcted hypertensive rats along with an associated reduction of serum angiotensin I-converting enzyme (ACE) activity. ACE activity in serum samples from captopril-treated animals that were assayed within 60 min after collection was inhibited 93%. However, this inhibition progressively decreased as the interval between time of assay and blood collection increased. This information would appear to be of considerable value in planning experiments for the determination of serum ACE activity from captopril-treated animals.

Aortic vascular and atrial responses to (±)‐1‐O‐octadecyl‐2‐acetyl‐glyceryl‐3‐phosphorylcholine

1 The effects of (±)‐1‐O‐octadecyl‐2‐acetyl‐glyceryl‐3‐phosphorylcholine (octadecyl‐AGPC) were studied in three types of aortic vascular smooth muscle preparations, namely, strips, rubbed and unrubbed rings, and an atrial preparation in normotensive rats. 2 In the resting tension state, octadecyl‐AGPC did not elicit significant contractions in either rubbed or unrubbed ring preparations at concentrations lower than 1 × 10−4m. However, at a concentration of 3 × 10−4 m, octadecyl‐AGPC markedly contracted both types of ring preparations. This contractile response was partially antagonized by pretreatment with reserpine and completely blocked by phentolamine (1 × 10−6m). 3 In preparations contracted with noradrenaline (NA), octadecyl‐AGPC elicited biphasic responses in intact ring preparations; an initial relaxation followed by contraction. Octadecyl‐AGPC induced only a slight contraction in strips and a slight relaxation in the rubbed ring preparation. 4 Octadecyl‐AGPC did not elicit any significant effect on chronotropy or inotropy at concentrations up to 3 × 10−5 m. When the concentration was 1 × 10−4 m, octadecyl‐AGPC produced significant positive chronotropic and inotropic effects on spontaneously beating right and electrically driven left atrial preparations, respectively. Both effects were blocked by propranolol (5 × 10−8m); reserpine pretreatment antagonized only the chronotropic response. 5 In [3H]‐dihydroalprenolol ([3H]‐DHA) binding studies, octadecyl‐AGPC had a Kd of 427.85 μm and thus was much less potent than isoprenaline (Kd = 465.10 nm) or propranolol (Kd = 4.4 nm) in displacing [3H]‐DHA in rat cardiac membrane preparations. 6 In conclusion, relaxation and contraction induced by octadecyl‐AGPC in aortic preparations is an indirect rather than a direct effect. An unknown factor released from endothelial cells is responsible for aortic smooth muscle relaxation by octadecyl‐AGPC while released NA appears to be responsible for aortic vascular contraction and for the positive chronotropic and inotropic effects in the atrial preparations.

A COMPARISON OF CARDIAC REACTIVITY AND β‐ADRENOCEPTOR NUMBER AND AFFINITY BETWEEN AORTA‐COARCTED HYPERTENSIVE AND NORMOTENSIVE RATS

1 The effects of noradrenaline (NA) and isoprenaline on isolated atria from aorta‐coarcted hypertensive rats (AHR) at early (6 day) and chronic (28 day) stages of hypertension were studied and compared with time‐matched, sham‐operated, normotensive rats (SNR). The number and affinity of β‐adrenoceptors ((−)‐[3H]‐dihydroalprenolol binding sites) were also studied in cardiac membranes prepared from these animals. 2 Six and 28 days after complete ligation of the abdominal aorta between the two renal arteries, rats became hypertensive with significantly greater arterial blood pressures than time‐matched SNR. 3 At both stages of hypertension, the atrial inotropic or chronotropic effects of NA and isoprenaline from hypertensive rats were similar to time‐matched SNR. Moreover, no differences in atrial reactivity were observed between the early and chronic stages of hypertension. 4 Irrespective of the stage of hypertension, cardiac membranes from the AHR contained the same number of β‐adrenoceptors as time‐matched SNR. In addition, the receptor affinity for the radioligand within each group was equivalent. However, the chronic stage hypertensive rats and their time‐matched controls contained fewer β‐adrenoceptors and these receptors had greater affinity for the radioligand when compared with cardiac membranes from rats at the early stage of hypertension and their controls. 5 The observed equivalent chronotropic and inotropic responses to NA and isoprenaline between the hypertensive and normotensive rats in both stages of hypertension may be explained in terms of similar receptor number and receptor binding affinity. 6 The reduced number of β‐adrenoceptors with greater binding affinity in day 28 normotensive or hypertensive rats may be a compensatory mechanism for these animals to maintain normal cardiac function with increasing age.

Studies on the effects of viprostol in isolated small blood vessels and thoracic aorta of the rat.

1 The effects of viprostol, prostaglandin E2 (PGE2) and nitroglycerin were studied in basilar artery, small mesenteric artery and the vein parallel to it as well as thoracic aorta of the rat. 2 In KCl‐contracted basilar artery, viprostol produced a concentration‐related biphasic response, contraction at concentrations < 3 × 10−6 m and relaxation at concentrations >3 × 10−6 m. PGE2 produced a concentration‐related contraction while nitroglycerin produced a concentration‐related relaxation. 3 In KCl‐contracted small mesenteric artery, viprostol produced a biphasic response which was similar to that in the basilar artery. PGE2 produced a contraction and nitroglycerin produced relaxation in a concentration‐dependent manner. 4 In KCl‐contracted small mesenteric vein, in contrast to basilar and mesenteric artery, viprostol produced only a concentration‐related relaxation in the range of 1 × 10−6 to 1 × 10−4 m. PGE2 produced a contraction and nitroglycerin produced a concentration‐related relaxation. 5 In KCl‐contracted thoracic aorta, PGE2 produced a biphasic response, relaxation at concentrations < 3 × 10−7 m and a concentration‐related contraction at concentrations > 3 × 10−7 m. Viprostol only produced a concentration‐related contraction at concentrations >1 × 10−6 m, which was significantly less in magnitude than the contraction produced by PGE2. Nitroglycerin produced a concentration‐related relaxation as seen in the small vessels. 6 In conclusion, the present data demonstrate that viprostol is a vasorelaxant agent which effectively dilates KCl‐contracted basilar, small mesenteric artery and vein, but not the thoracic aorta of rat. The potent antihypertensive action of viprostol is probably due to its relaxant effect on the small arteries and veins but not on the large conduit artery.

Effects of continuous angiotensin I-converting enzyme blockade on blood pressure, sympathetic activity and renin-angiotensin system in stroke-prone spontaneously hypertensive rats

Abstract The purpose of this study was to examine the effects of continuous angiotensin converting enzyme (ACE) blockade in stroke-prone spontaneously hypertensive rats (sp-SHR) on the renin-angiotensin system and on sympathetic activity. The pressor response to angiotensin II (AII) and norepinephrine (NE) were also examined after chronic blockade of ACE and compared to that of saline-treated controls. Captopril treatment had no effect on body weight. Serum ACE was significantly reduced on day 1; an effect that persisted through day 6 and day 10. Plasma renin activity (PRA) was elevated significantly on day 1 and remained at this high level throughout the 10 day observation period. Plasma NE was not altered by the chronic ACE blockade except on day 1, where there was a slight elevation of plasma NE in both groups. Pressor responses to AII and NE were not changed after chronic captopril treatment. It is observed that chronic inhibition of the renin-angiotensin system with captopril in sp-SHR resulted in a reduction of blood pressure, reduced serum ACE activity and elevated PRA. The constant plasma NE levels suggest that chronic inhibition of the renin-angiotensin system does not affect sympathetic activity. This study also indicates that long term inhibition of ACE does not alter pressor responses to either AII or NE.

The vasorelaxant effect of (±)-15-deoxy-16-hydroxy-16(α/β)-vinyl-prostaglandin E2 (CL 115, 129) and its methyl ester (CL 115, 347) on the isolated ductus arteriosus preparation

Abstract CL 115,129 and its methyl ester, CL 115,347, were studied for their vasorelaxant effects and compared to that of prostaglandin (PG) E 2 and its methyl ester on isolated ductus arteriosus (DA) from fetal lambs and rabbits. CL 115,129 and CL 115,347 potently relaxed the oxygen-indomethacin constricted ductus in a concentration dependent manner. The threshold concentration was 1×10 −13 M and the estimated EC 50 s (M) were 6.9×10 −8 and 4.3×10 −8 , respectively, for CL 115,129 and CL 115,347. Also confirmed was the vasorelaxant ability of PGE 2 . These studies indicate that the CL compounds possess potent vasorelaxant effects on the DA although less potent than PGE 2 or its methyl ester.

Relaxant actions of viprostol and prostaglandin E2 on tracheal muscle and their effects on calcium influx

The relaxant effect of viprostol was studied in monkey and guinea-pig tracheal muscle rings in vitro and compared to that of prostaglandin E2 (PGE2), isoproterenol (ISO) and verapamil (guinea-pig trachea only). Viprostol, PGE2, ISO and verapamil produced a concentration-dependent relaxation of carbachol-contracted tracheal preparations. The rank order of potency in monkey trachea was viprostol = ISO greater than PGE2, while in guinea-pig treachea it was ISO greater than viprostol greater than PGE2 greater than verapamil. The relaxant effect of viprostol or PGE2 was not antagonized by propranolol, suggesting that beta-adrenoceptors are not involved. Epithelium removal did not affect the bronchorelaxant effects of viprostol, PGE2 or ISO. In K+-rich, Ca++-free Krebs solution, preincubation with an IC30 of verapamil antagonized CaCl2-induced contractions while an IC30 of viprostol, PGE2 or ISO did not. Preincubation with an IC90 of viprostol, PGE2 or ISO produced 0.5, 0.5 and 1.0 log unit shifts to the right of the CaCl2 concentration response curves, respectively. At this concentration, viprostol did not reduce the maximum effect of CaCl2, but PGE2 and ISO reduced it approximately 20%. However, preincubation with an IC90 of verapamil completely abolished the CaCl2 contraction. In conclusion, viprostol is a potent bronchodilator whose effect does not depend on the epithelium, beta-adrenoceptors or antagonism of Ca++ influx. Whether the bronchodilator effect of viprostol is via intracellular sequestration of calcium as that of PGE2 remains to be studied.

Effects of CL 115,347, (±)-15-deoxy-16-vinyl-PGE2 methyl ester, on cardiovascular responses and plasma catecholamines in pithed stroke-prone spontaneously hypertensive rats during sympathetic stimulation

The effect of CL 115,347, a topically active antihypertensive PGE2 analog, and PGE2 on changes in blood pressure (BP), heart rate (HR) response and plasma epinephrine (E) and norepinephrine (NE) levels induced by stimulation of the sympathetic spinal cord outflow were studied in pithed stroke-prone spontaneously hypertensive rats (SHRSP). Surgical pithing significantly reduced plasma E but not NE levels suggesting that the sympathoadrenal medullary system differentially affects E and NE release. Sympathetic stimulation of the spinal cord of pithed SHRSP increased HR, BP, plasma E and NE levels. Topically applied CL 115,347 (0.001-0.2 mg/kg) dose-dependently decreased BP, while intravenously infused PGE2 (30 micrograms/kg/min) did not alter BP except for a brief initial drop. Topical application of CL 115,347 (0.1 mg/kg) also inhibited BP responses to sympathetic stimulation without effects on HR or plasma E or NE levels. Intravenous infusion of PGE2 (30 micrograms/kg/min) inhibited both BP and HR responses to spinal cord stimulation but did not alter plasma catecholamine levels. These studies in SHRSP suggest that CL 115,347 and PGE2 modulate cardiovascular responses mainly via postjunctional effects, but act differently on the cardiovascular elements, viz. CL 115,347 acts primarily on blood vessels while PGE2 acts on blood vessels and heart.