Nisoldipine and perfusion of post-stenotic myocardium in conscious pigs with different degrees of concentric stenosis

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Myocardial glucose uptake after dobutamine stress in chronic hibernating swine myocardium

BACKGROUND

In patients with hibernating myocardium, regional uptake of the glucose analog 2-fluorine 18-fluoro-2-deoxy-d-glucose (FDG) is increased under resting conditions. It is unclear whether the degree of increased FDG uptake correlates with the degree of impaired blood flow response and whether chronic changes in the glucose transporters may play a role in the enhanced FDG uptake under fasted conditions.

METHODS AND RESULTS

Twelve swine were instrumented with a constrictor on the left anterior descending (LAD) artery. Serial echocardiography and positron emission tomography studies were done to assess temporal changes in myocardial function, blood flow, and FDG uptake. One week after surgery (early study), wall thickening, blood flow, and postdobutamine FDG uptake in LAD and remote territories were similar. By approximately 6 weeks (late study), baseline wall thickening in the LAD region was lower than in remote regions (20% +/- 7% and 36% +/- 6%, P <.05), as was dobutamine-stimulated blood flow (0.92 +/- 0.16 mL. min(-1). g(-1) and 1.17 +/- 0.20 mL. min(-1). g(-1) in LAD and remote regions, respectively; P <.05). After the dobutamine infusion, FDG uptake in the LAD region during fasted conditions was higher than in remote regions (0.128 +/- 0.053 micromol. min(-1). g(-1) and 0.098 +/- 0.044 micromol. min(-1). g(-1), respectively; P <.05), and the increase was proportional to the impairment in dobutamine blood flow (r(2) = 0.62, P <.001). After the animals were killed, the LAD region showed a higher content of GLUT4 by immunoblots and a greater degree of translocation as estimated by immunohistochemistry. In 5 additional hibernating pigs studied under resting fasted conditions, FDG uptake and GLUT4 translocation were also higher in the LAD region, in the absence of dobutamine stimulation.

CONCLUSIONS

In hibernating myocardium, regional FDG uptake under fasting conditions is higher than in remote regions, both at rest and after an infusion of dobutamine. The degree of poststress FDG uptake is proportional to the impaired stress-induced blood flow. Total GLUT4 content as well as membrane-bound protein is higher in the hibernating tissue, and these changes may facilitate the observed increase in FDG uptake.

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.

Late preconditioning against myocardial stunning. An endogenous protective mechanism that confers resistance to postischemic dysfunction 24 h after brief ischemia in conscious pigs

Conscious pigs underwent a sequence of 10 2-min coronary occlusions, each separated by 2 min of reperfusion, for three consecutive days (days 1, 2, and 3 of stage I). The recovery of systolic wall thickening (WTh) after the 10th reperfusion was markedly improved on days 2 and 3 compared with day 1, indicating that the myocardium had become preconditioned against "stunning." 10 d after stage I, pigs underwent again a sequence of 10 2-min coronary occlusions for two consecutive days (days 1 and 2 of stage II). On day 1 of stage II, the recovery of WTh after the 10th reperfusion was similar to that noted on day 1 of stage I; on day 2 of stage II, however, the recovery of WTh was again markedly improved compared with day 1. Blockade of adenosine receptors with 8-p-sulfophenyl theophylline failed to prevent the development of preconditioning against stunning. Northern blot analysis demonstrated an increase in heat stress protein (HSP) 70 mRNA 2 h after the preconditioning ischemia; at this same time point, immunohistochemical analysis revealed a concentration of HSP70 in the nucleus and an overall increase in staining for HSP70. 24 h after the preconditioning ischemia, Western dot blot analysis demonstrated an increase in HSP70. This study indicates the existence of a new, previously unrecognized cardioprotective phenomenon. The results demonstrate that a brief ischemic stress induces a powerful, long-lasting (at least 48 h) adaptive response that renders the myocardium relatively resistant to stunning 24 h later (late preconditioning against stunning). This adaptive response disappears within 10 d after the last ischemic stress but can be reinduced by another ischemic stress. Unlike early and late preconditioning against infarction, late preconditioning against stunning is not blocked by adenosine receptor antagonists, and therefore appears to involve a mechanism different from that of other forms of preconditioning currently known. The increase in myocardial HSP70 is compatible with, but does not prove, a role of HSPs in the pathogenesis of this phenomenon.

Ro 40-5967, a novel calcium channel antagonist, protects against ventricular fibrillation

Ro 40-5967 is a new calcium channel antagonist that binds at the same membrane sites as verapamil, yet has minimal negative inotropic effects. The effects of Ro 40-5967 on the susceptibility to ventricular fibrillation were investigated and compared to diltiazem. Ventricular fibrillation (VF) was induced in 40 mongrel dogs with healed myocardial infarctions by a 2-min coronary occlusion during exercise. Twenty-four animals were found to be susceptible to VF and were given the treatments described below. Pretreatment with Ro 40-5967 (n = 17, 1000 micrograms/kg i.v.) significantly (P < 0.001) reduced the incidence of VF (13 of 17 protected) during the exercise plus ischemia test. Diltiazem (n = 8, 1000 micrograms/kg) completely suppressed VF. Lower doses of diltiazem and Ro 40-5967 did not prevent VF. The hemodynamic effects of Ro 40-5967 were also compared to diltiazem and verapamil. Diltiazem and verapamil, but not Ro 40-5967, increased P-R interval in a dose-dependent manner. Even when reflex tachycardia was controlled by beta-adrenoceptor blockade, Ro 40-5967 still exerted only minimal effects on P-R interval. Verapamil, but neither Ro 40-5967 nor diltiazem, provoked a dose-dependent negative inotropic response. All three drugs elicited large increases in coronary blood flow. These data support the hypothesis that calcium entry may play a critical role in the development of malignant arrhythmias during ischemia. Further, Ro 40-5967 can protect against ventricular fibrillation without significant negative inotropic or dromotropic effects.