William A. Herzer

Cardiovascular and renal effects of blocking A1 adenosine receptors.

The primary objective of this study was to test the hypothesis that endogenous adenosine/A1 receptor interactions participate in the normal regulation of the cardiovascular system and kidneys. In anesthetized rats, we examined the effects of pharmacologically equivalent doses of DPCPX and FK453 (two structurally dissimilar A1 receptor antagonists) and the effects of FR113452 (the less active enantiomer of FK.453) on 20 hemodynamic and renal function parameters. After baseline measurements, animals were randomized to an intravenous infusion of either vehicle or DPCPX, FK453, or FR113452 (10, 30, and 30 μg/kg/min, respectively), and all measurements were repeated. Neither DPCPX, FK453, nor FR113452 affected significantly the heart rate; cardiac output; stroke volume; arterial blood pressure; total vascular resistance; regional blood flows to and regional vascular resistances of the carotid, mesenteric, renal, and hind-quarter vascular beds; glomerular filtration rate; filtration fraction; or potassium excretion. However, DPCPX and FK453, but not FR113452, caused a pronounced diuresis (from 194 ± 30 to 696 ± 144 μl/30 min for DPCPX, and from 386 ± 76 to 814 ± 156 μl/30 min for FK453) and natriuresis (from 0.88 ± 0.30 to 5.09 ± 1.28 μmol/min for DPCPX, and from 1.58 ± 0.56 to 5.95 ± 1.02 μmol/min for FK453). These results firmly establish that (a) endogenous adenosine/A1 receptor interactions regulate renal excretory function, (b) this regulation is via a renal tubular, rather than hemodynamic, mechanism, and (c) A1 receptor antagonists cause diuresis without disturbing potassium homeostasis.

Angiotensin II/prostaglandin I2 interactions in spontaneously hypertensive rats.

The purpose of this study was to compare the effects of prostaglandin I2 on several cardiovascular parameters and to compare the ability of prostaglandin I2 to modify angiotensin II-induced changes in these cardiovascular parameters in spontaneously hypertensive versus normotensive rats. Studies were conducted in adult, age-matched, indomethacin-, and captopril-pretreated spontaneously hypertensive and normotensive rats that had been prepared for assessment of arterial blood pressure, cardiac output (thermodilution), and renal and mesenteric blood flows (transit-time flow probes). In both normotensive and hypertensive rats, intravenous infusions of prostaglandin I2 (0.003, 0.03, 0.3, 1, 3, and 10 micrograms/kg per minute) dose-dependently reduced mean arterial blood pressure, total peripheral resistance, and mesenteric vascular resistance but not renal vascular resistance. Only minor differences were detected between normotensive versus hypertensive rats with regard to the effects of prostaglandin I2 on baseline cardiovascular parameters (ie, in the absence of angiotensin II). In both rat strains, an intravenous infusion of angiotensin II (300 ng/kg per minute) increased mean arterial blood pressure, total peripheral resistance, and renal and mesenteric vascular resistances, and these effects of angiotensin II were similar in the two strains in the absence of prostaglandin I2. In both strains, prostaglandin I2 inhibited angiotensin II-induced changes in mean arterial blood pressure, total peripheral resistance, and renal and mesenteric vascular resistances. However, in the renal, but not mesenteric, vasculature of hypertensive rats, the ability of prostaglandin I2 to attenuate angiotensin II-induced vasoconstriction was strikingly reduced. These results indicate that although in general spontaneously hypertensive rats respond normally to prostaglandin I2, in the kidney of spontaneously hypertensive rats the ability of prostaglandin I2 to attenuate angiotensin II-induced vasoconstriction is reduced. This selective renal defect may relate to the pathogenesis of high blood pressure in this genetic model of hypertension.

Pertussis toxin-sensitive G-proteins and regulation of blood pressure in the spontaneously hypertensive rat.

1. Increased Gi‐protein‐mediated receptor–effector coupling in the vasculature of the spontaneously hypertensive rat (SHR) has been proposed as a contributing factor in the maintenance of elevated blood pressure. If increased Gi‐protein‐mediated activity plays an important role in hypertension in SHR, then inhibition of Gi‐proteins by pertussis toxin would be expected to decrease blood pressure in this genetic hypertensive model. To address this hypothesis, studies were undertaken comparing the cardiovascular effects of pertussis toxin in SHR and normotensive Wistar‐Kyoto (WKY) rats.

A comparison of the natriuretic/diuretic effects of rat vs. human leptin in the rat

Intrarenal artery infusions of low-dose human, but not mouse, leptin cause diuresis/natriuresis in rats [E. K. Jackson and P. Li. Am. J. Physiol. 272 ( Renal Physiol. 41): F333-F338, 1997]. The lack of effect of mouse leptin in the rat could be due to slight differences in the primary structure of mouse vs. rat leptin. To test this hypothesis, we infused single doses of rat (0.1, 0.3, 1, or 3 μg/min) or human (3 μg/min) leptin into the renal artery of rats for 140 min while continuously measuring blood pressure and the renal excretion rate of urine and electrolytes. Intrarenal infusions of rat leptin did not alter any measured parameter. Human leptin caused a delayed diuresis/natriuresis ( P < 0.0006 and P < 0.0049, respectively) that required ∼2 h to achieve a maximum effect and that was not accompanied by changes in blood pressure or potassium excretion. We conclude that low-dose human, but not low-dose rodent, leptin has direct diuretic/natriuretic activity. Our results can be explained from an evolutionary perspective, since obesity-induced hypertension would be a much greater selective force in hominids compared with rodents.Intrarenal artery infusions of low-dose human, but not mouse, leptin cause diuresis/natriuresis in rats [E. K. Jackson and P. Li. Am. J. Physiol. 272 (Renal Physiol. 41): F333-F338, 1997]. The lack of effect of mouse leptin in the rat could be due to slight differences in the primary structure of mouse vs. rat leptin. To test this hypothesis, we infused single doses of rat (0.1, 0.3, 1, or 3 microgram/min) or human (3 microgram/min) leptin into the renal artery of rats for 140 min while continuously measuring blood pressure and the renal excretion rate of urine and electrolytes. Intrarenal infusions of rat leptin did not alter any measured parameter. Human leptin caused a delayed diuresis/natriuresis (P < 0.0006 and P < 0.0049, respectively) that required approximately 2 h to achieve a maximum effect and that was not accompanied by changes in blood pressure or potassium excretion. We conclude that low-dose human, but not low-dose rodent, leptin has direct diuretic/natriuretic activity. Our results can be explained from an evolutionary perspective, since obesity-induced hypertension would be a much greater selective force in hominids compared with rodents.

3-Isobutyl-1-Methylxanthine decreases renal cortical interstitial levels of adenosine and inosine

The purpose of this study was to test the hypothesis that endogenous cyclic AMP, via metabolism by phosphodiesterase, contributes to interstitial levels of adenosine in the renal cortex in vivo. This hypothesis was tested by determining the effects of 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, on renal cortical interstitial levels of adenosine and inosine. Changes in renal cortical interstitial adenosine and inosine levels were assessed in rats by implanting microdialysis probes into the renal cortex and measuring adenosine and inosine levels in the dialysate exiting the kidney using high performance liquid chromatography. When added to the dialysate entering the kidney at concentrations of 0.5, 1 and 2.5 mM, 3-isobutyl-1-methylxanthine significantly and dose-dependently decreased interstitial levels of both adenosine and inosine. The percentage changes from baseline of interstitial levels of adenosine and inosine were: -39 +/- 6% and -19 +/- 6%, respectively, with 0.5 mM 3-isobutyl-1-methylxanthine; -45 +/- 7% and -24 +/- 8%, respectively, with 1 mM 3-isobutyl-1-methylxanthine; and -56 +/- 12% and -38 +/- 8%, respectively, with 2.5 mM 3-isobutyl-1-methylxanthine. These data suggest that in the renal cortex, cyclic AMP metabolism via phosphodiesterase is an important source of renal interstitial adenosine.

Enhanced interaction between renovascular α2-Adrenoceptors and angiotensin II receptors in genetic hypertension

In spontaneously hypertensive rats (SHR), hypertension is mediated in part by an enhanced renovascular response to angiotensin (Ang) II. Pertussis toxin normalizes renovascular responses to Ang II and lowers blood pressure in SHR, suggesting a role for altered Gi signaling in the enhanced renovascular response to Ang II in SHR. To further investigate this hypothesis, we measured reductions in renal blood flow and increases in renovascular resistance in response to intrarenal infusions of Ang II in the presence and absence of coactivation of &agr;2-adrenoceptors (ie, receptors selectively coupled to Gi) with UK 14,304 in adrenalectomized, renal-denervated, captopril-pretreated SHR and normotensive Wistar-Kyoto rats. In SHR, but not Wistar-Kyoto rats, UK 14,304 markedly enhanced renovascular responses to Ang II and vasopressin. However, UK 14,304 did not enhance renovascular responses to methoxamine (&agr;1-adrenoceptor agonist) in either strain. In uninephrectomized, normotensive Sprague-Dawley animals and in Sprague-Dawley rats with nongenetic hypertension induced by uninephrectomy, chronic administration of deoxycorticosterone acetate, and 1% saline as drinking water, UK 14,304 had little or no effect on renovascular responses to Ang II. In SHR, intrarenal infusions of U73122, a phospholipase C/D inhibitor, blocked the enhancement of renovascular responses to Ang II by UK 14,304. We conclude that activation of &agr;2-adrenoceptors selectively enhances renovascular responses to Ang II and vasopressin in vivo in animals with genetic hypertensive but not in normotensive animals or animals with acquired hypertension. These results suggest that in SHR, there is a genetically mediated enhanced cross talk between the Gi signal transduction pathway and signal transduction pathways activated by Ang II and vasopressin, but not methoxamine, and involving phospholipase C and/or D.

Defective modulation of angiotensin II-induced renal vasoconstriction in hypertensive rats.

In a previous study we observed that the ability of intravenous infusions of prostaglandin I2 to attenuate vasoconstriction caused by intravenous infusions of angiotensin II was reduced in the renal but not mesenteric vasculature of spontaneously hypertensive rats (SHR). One objective of the current study was to determine whether a renal defect in the angiotensin II/prostaglandin I2 interaction in SHR could be confirmed even when confounding hemodynamic changes induced by intravenous infusions of prostaglandin I2 were avoided. The second objective was to determine whether abnormal modulation of angiotensin II-induced renal vasoconstriction was present even in SHR that were maintained normotensive from an early age. Four-week-old SHR and normotensive Wistar-Kyoto rats were randomized to receive either normal drinking water or drinking water containing captopril (100 mg/kg per day). At 14 to 18 weeks of age, rats were pretreated with indomethacin to block the production of endogenous prostaglandin I2, and changes in mesenteric and renal vascular resistances induced by suprarenal/supramesenteric aortic infusions of angiotensin II (10, 30, and 100 ng/kg per minute) were elicited in the presence and absence of aortic infusions of prostaglandin I2 (0.1 and 0.3 micrograms/kg per minute). Data were analyzed globally using four- and three-factor ANOVAs. The ability of prostaglandin I2 to attenuate the renal vasoconstrictor response to angiotensin II was strain specific (P = .0138), and this strain-specific interaction was not influenced by chronic treatment with captopril (P = .3526).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of type IV phosphodiesterase inhibition on cardiac function in the presence and absence of catecholamines.

Type IV phosphodiesterase (PDE4) inhibitors may be useful in several diseases in which catecholamine infusions are commonly used, including asthma, sepsis, and multiple organ failure. To determine whether type IV phosphodiesterase inhibitors alter baseline or catecholamine-induced changes in cardiac function or both, we examined the effects of Ro 20-1724 (PDE4 inhibitor) on several cardiac-performance parameters in the absence and presence of norepinephrine, epinephrine, isoproterenol, and dobutamine infusions (3, 1, 0.1, and 3 microg/kg/min, respectively). Male Sprague-Dawley rats received either Ro 20-1724 (10 microg/kg/min; n = 7) or vehicle (n = 6). After a left ventricular catheter was placed and connected to a heart-performance analyzer, each animal received each of the four catecholamines in randomized order (10 min per catecholamine with a 30-min washout period between infusions). In the absence of catecholamines, Ro 20-1724 significantly but mildly (i.e., <10%) increased heart rate but did not alter significantly any other measured cardiac parameter. In addition, Ro 20-1724 did not significantly alter norepinephrine-, epinephrine-, or dobutamine-induced changes in cardiac-performance parameters. There was, however, a significant attenuation of the isoproterenol-induced increase in a single measure of cardiac contractility (maximum dP/dt normalized to pressure). PDE4 inhibition does not cause significant cardiac toxicities in rats, both in the absence and presence of catecholamines. Our data suggest that PDE4 inhibitors may be safely used in critically ill patients receiving catecholamines. A clinical trial of this family of drugs in patients with critical illness is now being planned.

Type IV phosphodiesterase inhibition improves cardiac contractility in endotoxemic rats.

Type IV phosphodiesterase inhibitors have a potential role in treating human sepsis. We examined the cardiac performance effects of type IV phosphodiesterase inhibition in vivo, in the absence and presence of catecholamines. Rats were randomized to receive either 4-(3-Butoxy-4-methoxybenzyl)imidazolidin-2-one (Ro 20-1724) at 0 (vehicle), 2 or 10 microg/kg/min. Utilizing a left ventricular catheter to measure cardiac performance, each animal received each of the two catecholamines, epinephrine and norepinephrine, in randomized order. Rats then received intravenous endotoxin and additional infusions of catecholamines. Ro 20-1724 at 2 microg/kg/min protected cardiac contractility during endotoxemia, and at 10 microg/kg/min increased cardiac contractility and protected cardiac function during endotoxemia. Neither dose interfered with the maximal contractile response to catecholamines. Type IV phosphodiesterase inhibition with Ro 20-1724 exerts beneficial effects on cardiac performance during septicemia in an in vivo animal model. Clinical studies of type IV phosphodiesterase inhibitors in critically ill patients are indicated.

Attenuation of noradrenergic neurotransmission by the thromboxane synthetase inhibitor, UK 38,485

The purpose of this study was to determine the effects of chronic administration of the thromboxane synthetase inhibitor, UK 38,485, on noradrenergic neurotransmission. Male Sprague Dawley rats (n = 14) were treated once daily with either UK 38,485 (100 mg/kg; n = 7) or the vehicle of UK 38,485 (olive oil; n = 7) by gavage. The dose of UK 38,485 chosen was sufficient to inhibit ex vivo platelet TXB2 production by greater than 90% for 24 hours. One week into the treatment animals were prepared for in situ perfusion of their mesenteric vascular beds. Vasoconstrictor responses to both exogenous norepinephrine and periarterial nerve stimulation were determined both before and during an infusion of angiotensin II (9 ng/min) into the superior mesenteric artery. UK 38,485 significantly (P less than 0.02) attenuated the vascular response to periarterial nerve stimulation without altering the vascular response to either norepinephrine or angiotensin II. UK 38,485 did not influence the baseline perfusion pressure, the mean arterial blood pressure or the potentiation of neurotransmission by angiotensin II. These data indicate that in the in situ rat mesentery UK 38,485 attenuates the release of neurotransmitter from sympathetic nerve terminals.