Haemodynamic profile of the potassium channel activator EMD 52692 in anaesthetized pigs

Angiotensin II production and distribution in the kidney: I. A kinetic model

Information on the levels of angiotensin II (Ang II) and its receptors in the various renal tissue compartments is still incomplete. A model is presented describing the kinetics of Ang II production, distribution, and disposal in the renal cortex. Basic features are: (1) the model is designed to derive, from Ang II measurements in blood and in whole tissue, estimates of the local densities of the Ang II type 1 (AT(1)) and type 2 (AT(2)) receptors, and to calculate the concentrations of endocrine and paracrine Ang II they actually 'see'; (2) glomerular and peritubular tissue are conceived as separate regions (glomerular region (Glom), peritubular region (Pt)); (3) in Glom and in Pt, Ang II is homogeneously distributed in capillary blood and in interstitial fluid; (4) the model allows for local Ang II concentration gradients between interstitium and blood; (5) Ang II from the circulation diffuses into the interstitium of Glom after convective transcapillary transport; (6) Ang II produced in tubules or Pt enters the microcirculation through diffusive overflow from interstitium; (7) the presence of cell-surface-bound Ang II depends on the reaction with AT(1) and AT(2) receptors, and the presence of intracellular Ang II depends on the internalization of Ang II - AT(1) receptor complex; and (8) the model provides for glomerular filtration, vasopeptidase-mediated degradation, and intracellular degradation as mechanisms of elimination. This model can serve as a framework for detailed quantitative studies of the renin-angiotensin system in the kidney.

Role of K(ATP)(+) channels in regulation of systemic, pulmonary, and coronary vasomotor tone in exercising swine

The role of ATP-sensitive K(+) (K(ATP)(+)) channels in vasomotor tone regulation during metabolic stimulation is incompletely understood. Consequently, we studied the contribution of K(ATP)(+) channels to vasomotor tone regulation in the systemic, pulmonary, and coronary vascular bed in nine treadmill-exercising swine. Exercise up to 85% of maximum heart rate increased body O(2) consumption fourfold, accommodated by a doubling of both cardiac output and body O(2) extraction. Mean aortic pressure was unchanged, implying that systemic vascular conductance (SVC) also doubled, whereas pulmonary artery pressure increased almost in parallel with cardiac output, so that pulmonary vascular conductance (PVC) increased only 25 +/- 9% (both P < 0.05). Myocardial O(2) consumption tripled during exercise, which was paralleled by an equivalent increase in O(2) supply so that coronary venous PO(2) was maintained. Selective K(ATP)(+) channel blockade with glibenclamide (3 mg/kg iv), decreased SVC by 29 +/- 4% at rest and by 10 +/- 2% at 5 km/h (both P < 0.05), whereas PVC was unchanged. Glibenclamide decreased coronary vascular conductance and hence myocardial O(2) delivery, necessitating an increase in O(2) extraction from 76 +/- 2% to 86 +/- 2% at rest and from 79 +/- 2% to 83 +/- 1% at 5 km/h. Consequently, coronary venous PO(2) decreased from 25 +/- 1 to 17 +/- 1 mmHg at rest and from 23 +/- 1 to 20 +/- 1 mmHg at 5 km/h (all values are P < 0.05). In conclusion, K(ATP)(+) channels dilate the systemic and coronary, but not the pulmonary, resistance vessels at rest and during exercise in swine. However, opening of K(ATP)(+) channels is not mandatory for the exercise-induced systemic and coronary vasodilation.

Activation of ATP-sensitive potassium channels decreases neuronal injury caused by chemical hypoxia

Cerebral ischemia is known to induce endogenous adaptive mechanisms such as the activation of ATP-sensitive potassium channels that can prevent or delay neuronal injury. This process can be therapeutically mimicked by treatment with potassium channel openers. Primary neuronal cell cultures were derived from embryonic chick telencephalon and were exposed to chemical hypoxia (1 mM cyanide) or excitotoxic injury (1 mM L-glutamate). While treatments with the potassium channel openers bimakalim (1-10 microM) and EMD 57283 (0.1-10 microM) were clearly able to maintain neuronal viability after chemical hypoxia, similar concentrations of the drugs had negligible effects on glutamate-induced neurotoxicity. In contrast, both types of neuronal injury were sensitive to the protective action of the glutamate receptor antagonist dizocilpine (MK-801; 0.1-1 microM). The neuroprotective effect of bimakalim against chemically induced hypoxic injury was reversed by tolbutamide (1 microM), an ATP-sensitive potassium channel blocker. These experiments demonstrate neuroprotective effects of potassium channel openers that could be related to inhibition of neurotransmitter release.

In swine myocardium, the infarct size reduction induced by U-89232 is glibenclamide sensitive: evidence that U-89232 is a cardioselective opener of ATP-sensitive potassium channels

We determined whether U-89232, a derivative of the ATP-sensitive potassium (KATP) channel opener cromakalim, is cardioselective and whether its action on the myocardium is still sensitive to glibenclamide. Experiments were performed in open-chest pigs subjected to a 60-min occlusion of the left anterior descending coronary artery (LADCA) and to 2 h of reperfusion. Four groups of animals were studied (n = 6 each). Animals received either U-89232, 3 mg/kg i.v. over a 15-min period (U), or glibenclamide, a selective KATP channel blocker, 1 mg/kg i.v. over a 15-min period (GLI) before the LADCA occlusion. In the GLI + U group, first glibenclamide (1 mg/kg/15 min) and then U-89232 (3 mg/kg/15 min) were infused before the 60 min of ischemia. Saline-treated animals served as controls (CON). Hemodynamic parameters were continuously monitored. Regional contractile wall function was quantified with ultrasonic crystals aligned to measure wall thickening. At the end of the protocol, infarct size (IS, as percentage of risk region) was determined by incubating the myocardium with p-nitrobluetetrazolium. With comparable myocardium at risk, infusion of U-89232 before 60 min of LADCA occlusion significantly reduced infarct size (IS, 18.5 +/- 3.7%; p < 0.001 vs. 63.2 +/- 3.3% for the controls), whereas glibenclamide had no effect on infarct size (IS, 69.5 +/- 4.4%). The administration of glibenclamide before U-89232 infusion blocked the infarct size-reducing effect of U-89232 [IS, 61.2 +/- 9.1 (NS) vs. controls and p < 0.001 vs. U]. Infusion of U-89232 had no effect on hemodynamic parameters or on regional wall function. At least in a pig model, U-89232 appears to be a cardioselective KATP channel opener, because in the absence of hemodynamic alterations, it exhibits a profound cardioprotective effect, which is fully reversible by blocking KATP channels.

Assessment of the role of the renin-angiotensin system in cardiac contractility utilizing the renin inhibitor remikiren

1. The role of the renin-angiotensin system in the regulation of myocardial contractility is still debated. In order to investigate whether renin inhibition affects myocardial contractility and whether this action depends on intracardiac rather than circulating angiotensin II, the regional myocardial effects of systemic (i.v.) and intracoronary (i.c.) infusions of the renin inhibitor remikiren, were compared and related to the effects on systemic haemodynamics and circulating angiotensin II in open-chest anaesthetized pigs (25-30 kg). The specificity of the remikiren-induced effects was tested (1) by studying its i.c. effects after administration of the AT1-receptor antagonist L-158,809 and (2) by measuring its effects on contractile force of porcine isolated cardiac trabeculae. 2. Consecutive 10 min i.v. infusions of remikiren were given at 2, 5, 10 and 20 mg min-1. Mean arterial pressure (MAP), cardiac output (CO), heart rate (HR), systemic vascular resistance (SVR), myocardial oxygen consumption (MVO2) and left ventricular (LV) dP/dtmax were not affected by remikiren at 2 and 5 mg min-1, and were lowered at higher doses. At the highest dose, MAP decreased by 48%, CO by 13%, HR by 14%, SVR by 40%, MVO2 by 28% and LV dp/dtmax by 52% (mean values; P < 0.05 for difference from baseline, n = 5). The decrease in MVO2 was accompanied by a decrease in myocardial work (MAP x CO), but the larger decline in work (55% vs. 28%; P < 0.05) implies a reduced myocardial efficiency ((MAP x CO)/MVO2). 3. Consecutive 10 min i.c. infusions of remikiren were given at 0.2, 0.5, 1, 2, 5 and 10 mg min-1. MAP, CO, MVO2 and LV dP/dtmax were not affected by remikiren at 0.2, 0.5 and 1 mg min-1, and were reduced at higher doses. At the highest dose, MAP decreased by 31%, CO by 26%, MVO2 by 46% and LV dP/dtmax by 43% (mean values; P < 0.05 for difference from baseline, n = 6). HR and SVR did not change at any dose. 4. Thirty minutes after a 10 min i.v. infusion of the AT1 receptor antagonist, L-158,809 at 1 mg min-1, consecutive 10 min i.c. infusions (n = 5) of remikiren at 2, 5 and 10 mg min-1 no longer affected CO and MVO2, and decreased LV dP/dtmax by maximally 27% (P < 0.05) and MAP by 14% (P < 0.05), which was less than without AT1-receptor blockade (P < 0.05). HR and SVR remained unaffected. 5. Plasma renin activity and angiotensin I and II were reduced to levels at or below the detection limit at doses of remikiren that were not high enough to affect systemic haemodynamics or regional myocardial function, both after i.v. and i.c. infusion. 6. Remikiren (10(-10) to 10(-4) M) did not affect contractile force of porcine isolated cardiac trabeculae precontracted with noradrenaline. In trabeculae that were not precontracted no decrease in baseline contractility was observed with remikiren in concentrations up to 10(-5) M, whereas at 10(-4) M baseline contractility decreased by 19% (P < 0.05). 7. Results show that with remikiren i.v., at the doses we used, blood pressure was lowered primarily by vasodilation and with remikiren i.c. by cardiac depression. The blood levels of remikiren required for its vasodilator action are lower than the levels affecting cardiac contractile function. A decrease in circulating angiotensin II does not appear to be the sole explanation for these haemodynamic responses. Data support the contention that myocardial contractility is increased by renin-dependent angiotensin II formation in the heart.

Role of K+ATP channels in coronary vasodilation during exercise

BACKGROUND

The mechanism of metabolic regulation of coronary vascular tone is still unclear. Therefore, we examined the role of vascular smooth muscle K+ATP channels in regulating coronary blood flow under resting conditions, during increments in myocardial metabolic demand produced by treadmill exercise, and in response to a brief ischemic stimulus.

METHODS AND RESULTS

Ten chronically instrumented dogs were studied at rest and during a four-stage exercise protocol under control conditions and during intracoronary infusion of the K+ATP channel blocker glibenclamide at rates of 10 and 50 micrograms.kg-1 x min-1. Glibenclamide (50 micrograms.kg-1 x min-1) decreased coronary blood flow at rest from 51 +/- 4 to 42 +/- 6 mL/min (P < .05), decreased myocardial oxygen consumption from 5.70 +/- 0.31 to 4.11 +/- 0.56 mL O2/min (P < .05), and decreased systolic wall thickening from 21 +/- 3% to 12 +/- 3% (P < .05). The depression of systolic wall thickening produced by glibenclamide was reversed when coronary blood flow was restored to the control level with intracoronary nitroprusside, indicating a primary effect of glibenclamide on coronary flow during resting conditions. However, glibenclamide did not impair the increases of coronary blood flow, myocardial oxygen consumption, and systolic wall thickening that occurred during exercise. In eight resting awake dogs, 50 micrograms.kg-1 x min-1 glibenclamide decreased the peak reactive hyperemia blood flow rate following a 20-second coronary occlusion from 149 +/- 14 mL/min during control conditions to 111 +/- 15 mL/min (P < .05), decreased the duration of reactive hyperemia from 49 +/- 6 to 33 +/- 3 seconds (P < .05), and decreased reactive hyperemia excess flow from 33 +/- 5 to 20 +/- 4 mL (P < .05).

CONCLUSIONS

These data demonstrate that K+ATP channels modulate coronary vasomotor tone under resting conditions and contribute to coronary vasodilation during ischemia. However, the coronary vasculature retains the capacity to dilate in response to increases in oxygen demand produced by exercise when K+ATP channels are blocked.

Cardiovascular effects of the novel potassium channel opener bimakalim in conscious pigs after myocardial infarction: a comparative study with nicorandil

The benzopyran-derivative bimakalim is an ATP-dependent potassium channel activator that has been shown to be a potent arterial vasodilator in anesthetized pigs. In the present study we evaluated the cardiovascular profile of bimakalim in normal conscious animals and conscious animals with chronic left ventricular dysfunction and compared the results to those obtained with nicorandil. In normal conscious pigs, bimakalim (37.5-300 ng/kg/min, n = 6) and nicorandil (10-80 micrograms/kg/min, n = 8) increased cardiac output from 2.7 +/- 0.1 l/min to 3.8 +/- 0.2 l/min and from 2.7 +/- 0.1 l/min to 3.9 +/- 0.3 l/min (both p less than .05) due to increases in heart rate (up to 62 +/- 14% and 74 +/- 9%, respectively, both p less than .05). The mean arterial blood pressure decreased gradually from 104 +/- 4 mmHg to 91 +/- 5 mmHg with bimakalim and from 98 +/- 3 mmHg to 84 +/- 5 mmHg with nicorandil (both p less than .05), due to similar decreases in systemic vascular resistance. LVdP/dtmax also increased with both drugs (up to 48 +/- 11% and 69 +/- 7%, respectively, p less than .05), but left ventricular end-diastolic pressure remained unchanged with bimakalim, while it gradually decreased from 9 +/- 1 mmHg to 5 +/- 1 mmHg (p less than .05) with nicorandil. In pigs with a 3- to 4-week-old myocardial infarction, the vasodilator responses to bimakalim (n = 8) and nicorandil (n = 9) were not affected, but the increases in heart rate and LVdP/dtmax were attenuated compared to the effects in the normal animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against a role for dopamine D1 receptors in the myocardium of the pig

1. We investigated the presence of dopamine D1 receptors in the myocardium of anesthetized pigs using intravenous infusions of dopamine, alone and after alpha- and beta-adrenoceptor blockade and intracoronary infusions of the selective D1 receptor agonist, fenoldopam. 2. Intravenous infusion of dopamine (2.5, 5 and 10 micrograms kg-1 min-1 for 10 min, n = 6) caused dose-dependent changes in heart rate (from 94 +/- 6 to 132 +/- 10 beats min-1, P less than 0.05), the maximal rate of rise of left ventricular pressure (LVdP/dtmax; from 2280 +/- 170 to 4800 +/- 410 mmHgs-1, P less than 0.05), mean arterial blood pressure (from 87 +/- 5 to 62 +/- 3 mmHg) and systemic vascular resistance (from 40 +/- 4 to 28 +/- 2 mmHgl-1 min, P less than 0.05). The increases in heart rate and LVdP/dtmax were abolished when dopamine was infused after alpha- and beta-adrenoceptor blockade. The vasodilator response was, however, only minimally affected. 3. Intravenous infusions of dopamine decreased coronary vascular resistance from 0.90 +/- 0.06 to 0.53 +/- 0.07 mmHg ml-1 min 100 g (P less than 0.05). This action of dopamine was not observed when dopamine was infused after blockade of the alpha- and beta-adrenoceptors. 4. Pretreatment with alpha- and beta-adrenoceptor blockade had no effect or only slightly attenuated the dopamine-induced decrease in vascular resistance of the brain, kidneys, adrenals and small intestine. 5. In 7 animals, intracoronary doses of 0.04, 0.1, 0.2 and 0.4g kg1- min 1 of fenoldopam had no effect on coronary venous oxygen content, local myocardial oxygen consumption, coronary blood flow or coronary vascular resistance. However, systemic effects were observed at the highest two doses, as manifested by a drop in mean arterial blood pressure from 82 +/- 4 to 72 +/- 4mmHg (P < 0.05) due to peripheral vasodilatation (e.g. cerebral vascular bed). Heart rate, LVdP/dt,.,, regional myocardial segment length shortening and left ventricular end-diastolic pressure were not affected at these doses. In 2 animals the infusion rate was increased to 4jug kg1 min 1, but again there was no evidence for coronary vasodilatation. 6. We conclude that the intravenous infusion of dopamine after alpha- and beta-adrenoceptor blockade and the intracoronary infusion of fenoldopam provided no evidence for a major role of D1 receptors in the coronary circulation of pigs. The absence of any effect of the employed doses of fenoldopam on LVdP/dt.mx and on regional myocardial segment length shortening also indicates that fenoldopam does not exhibit any inotropic action in this species.