Altered Ca2+ handling and myofilament desensitization underlie cardiomyocyte performance in normothermic and hyperthermic heat-acclimated rat hearts

Heat acclimation (AC) improves cardiac mechanical and metabolic performance. Using cardiomyocytes and isolated hearts from 30-day and 2-day acclimated rats (AC and AC-2d, 34°C), we characterized cellular contractile mechanisms under normothermic (37°C) and hyperthermic (39–42°C) conditions. To determine contractile responses, Ca2+ transients (Ca2+ T), sarcoplasmic reticulum (SR) Ca2+ pool size (fura-2/indo-1 fluorescence), force generation [amplitude systolic motion (ASM)], L-type Ca2+ channels [dihydropyridine receptor (DHPR)], ryanodine receptors (RyRs), and total (PLBt) and phosphorylated phospholamban [serine phosphorylated (PLBs) and theonine phosphorylated (PLBtr)] proteins and transcripts were measured (Western blot, RT-PCR). Cardiac mechanical performance was measured using a Langendorff system. We demonstrated that AC and AC-2d increased Ca2+ T amplitude (148% and 147%, respectively) and twitch force (180% and 130%, respectively) and desensitized myofilaments, as indicated by a rightward shift in the ASM-Ca2+ relationships, despite no change in SR Ca2+ pool size. Hence, generation of higher Ca2+ T underlies greater force development in AC and AC-2d myocytes. In isolated hearts, ryanodine administration eliminated differences between AC and control (C) hearts, implying an important role for RyRs in that acclimation phase. Increased expression of DHPR and RyRs, and decreased PLBs/PLBt in AC hearts only, suggest that different pathways increase force generation in the AC-2d vs. AC myocytes. At basal beating rates, hyperthermia (39–41°C) enhanced pressure generation in AC hearts. C hearts failed to restitute pressure beyond 39°C. Increased beating frequency produced negative inotropic response. In C cardiomyocytes, hyperthermia elevated basal cytosolic Ca2+ and tension, Ca2+ T, and ASM. AC myocytes enhanced Ca2+ T but showed myofilament desensitization, suggesting its involvement in cardiac protection against hyperthermia. Collectively, both Ca2+ turnover and myofilament responsiveness are important adaptive acclimatory targets during normothermic and hyperthermic conditions.

Magnesium deficiency augments myocardial response to reactive oxygen species

Magnesium (Mg) deficiency and oxidative stress are independently implicated in the etiopathogenesis of various cardiovascular disorders. This study was undertaken to examine the hypothesis that Mg deficiency augments the myocardial response to oxidative stress. Electrically stimulated rat papillary muscle was used for recording the contractile variation. Biochemical variables of energy metabolism (adenosine triphosphate (ATP) and creatine phosphate) and markers of tissue injury (lactate dehydrogenase (LDH) release and lipidperoxidation), which can affect myocardial contractility, were assayed in Langendorff-perfused rat hearts. Hydrogen peroxide (100 µmol/L) was used as the source of reactive oxygen species. The negative inotropic response to H2O2 was significantly higher in Mg deficiency (0.48 mmol Mg/L) than in Mg sufficiency (1.2 mmol Mg/L). Low Mg levels did not affect ATP levels or tissue lipid peroxidation. However, H2O2 induced a decrease in ATP; enhanced lipid peroxidation and the release of LDH were augmented by Mg deficiency. Increased lipid peroxidation associated with a decrease in available energy might be responsible for the augmentation of the negative inotropic response to H2O2 in Mg deficiency. The observations from this study validate the hypothesis that myocardial response to oxidative stress is augmented by Mg deficiency. This observation has significance in ischemia–reperfusion injury, where Mg deficiency can have an additive effect on the debilitating consequences.

α-Adrenoceptor stimulation-mediated negative inotropism and enhanced Na+/Ca2+ exchange in mouse ventricle

Mechanisms underlying the negative inotropic response to α-adrenoceptor stimulation in adult mouse ventricular myocardium were studied. In isolated ventricular tissue, phenylephrine (PE), in the presence of propranolol, decreased contractile force by ∼40% of basal value. The negative inotropic response was similarly observed under low extracellular Ca2+ concentration ([Ca2+]o) conditions but was significantly smaller under high-[Ca2+]o conditions and was not observed under low-[Na+]o conditions. The negative inotropic response was not affected by nicardipine, ryanodine, ouabain, or dimethylamiloride (DMA), inhibitors of L-type Ca2+ channel, Ca2+ release channel, Na+-K+ pump, or Na+/H+exchanger, respectively. KB-R7943, an inhibitor of Na+/Ca2+ exchanger, suppressed the negative inotropic response mediated by PE. PE reduced the magnitude of postrest contractions. PE caused a decrease in duration of the late plateau phase of action potential and a slight increase in resting membrane potential; time courses of these effects were similar to that of the negative inotropic effect. In whole cell voltage-clamped myocytes, PE increased the L-type Ca2+ and Na+/Ca2+ exchanger currents but had no effect on the inwardly rectifying K+, transient outward K+, or Na+-K+-pump currents. These results suggest that the sustained negative inotropic response to α-adrenoceptor stimulation of adult mouse ventricular myocardium is mediated by enhancement of Ca2+ efflux through the Na+/Ca2+ exchanger.

Characterization of responses to neurokinin A in the isolated perfused guinea pig heart

Goals of this study were to identify and characterize effects of neurokinin A (NKA) in isolated guinea pig hearts. Bradycardia, augmentation of ventricular contractions, and reduction of perfusion pressure were prominent responses to bolus injections of NKA (0.25–25 nmol). NKA was more potent than substance P (SP) in causing bradycardia but did not differ in potency for lowering perfusion pressure. Doses of SP of 25 nmol or less decreased ventricular force, whereas 100 nmol caused a biphasic response. The percent decrease in heart rate produced by 25 nmol NKA was reduced from 58.0 ± 4.8 to 39.6 ± 3.5% in the presence of 1 μM atropine ( n = 5). The positive inotropic response to 25 nmol of NKA in spontaneously beating hearts was replaced by a negative inotropic response during pacing (22.5 ± 3.3% increase vs. 11.7 ± 1.7% decrease, n = 5). Reserpine pretreatment did not affect the positive inotropic response to NKA. Specific binding sites for 125I-labeled NKA were localized to intracardiac ganglia and coronary arteries but not to myocardium. It was concluded that 1) negative chronotropic responses to NKA involve cholinergic and noncholinergic mechanisms, and 2) the positive inotropic response is an indirect action.

Beta-blockade reduces effects of adenosine and carbachol by transregulation of inhibitory receptors and Gi proteins

Chronic blockade of stimulatory beta-adrenergic receptors may decrease inhibitory receptors of the adrenergic signal transduction system. This transregulation process might reduce the negative inotropic response of the myocardium to inhibitory receptor stimulation. Rats were treated for 6 days with the beta-blocker atenolol (2 mg/day). beta-Adrenergic receptors in cardiac plasma membranes increased from 49 +/- 6 to 75 +/- 9 fmol/mg protein (means +/- SE; P = 0.053), whereas muscarinic M2 receptors decreased (155 +/- 15 vs. 105 +/- 10 fmol/mg protein; P < or = 0.05). Moreover, inhibitory G alpha(i) proteins were reduced by 36%. The functional responses of isolated hearts to inhibitory agonists after prestimulation with isoproterenol (3 nmol/l) were significantly blunted. The Ki value for the negative inotropic response of the maximal rise in developed left ventricular pressure (dP/dt(max)) to adenosine (0.1-100 micromol/l) increased from 5.9 +/- 1.7 to 24.0 +/- 2.5 micromol/l (P < or = 0.001). A similar rightward shift of the dose-response curve was observed for the effects of adenosine on developed left ventricular pressure (LVP) and of carbachol (0.01-10 micromol/l) on LVP and dP/dt. Thus chronic beta-blockade leads to a coordinate transregulation of inhibitory receptors and Gi proteins, reducing the effects of inhibitory receptor activation of the heart. This mechanism may contribute to the beneficial effects of beta-blocker therapy in heart failure.