The effects of temperature upon calcium exchange in intact cultured cardiac myocytes

Store-operated Ca2+ entry supports contractile function in hearts of hibernators

Hibernators have a distinctive ability to adapt to seasonal changes of body temperature in a range between 37°C and near freezing, exhibiting, among other features, a unique reversibility of cardiac contractility. The adaptation of myocardial contractility in hibernation state relies on alterations of excitation contraction coupling, which becomes less-dependent from extracellular Ca²⁺ entry and is predominantly controlled by Ca²⁺ release from sarcoplasmic reticulum, replenished by the Ca²⁺-ATPase (SERCA). We found that the specific SERCA inhibitor cyclopiazonic acid (CPA), in contrast to its effect in papillary muscles (PM) from rat hearts, did not reduce but rather potentiated contractility of PM from hibernating ground squirrels (GS). In GS ventricles we identified drastically elevated, compared to rats, expression of Orai1, Stim1 and Trpc1/3/4/5/6/7 mRNAs, putative components of store operated Ca²⁺ channels (SOC). Trpc3 protein levels were found increased in winter compared to summer GS, yet levels of Trpc5, Trpc6 or Trpc7 remained unchanged. Under suppressed voltage-dependent K⁺, Na⁺ and Ca²⁺ currents, the SOC inhibitor 2-aminoethyl diphenylborinate (2-APB) diminished whole-cell membrane currents in isolated cardiomyocytes from hibernating GS, but not from rats. During cooling-reheating cycles (30°C–7°C–30°C) of ground squirrel PM, 2-APB did not affect typical CPA-sensitive elevation of contractile force at low temperatures, but precluded the contractility at 30°C before and after the cooling. Wash-out of 2-APB reversed PM contractility to control values. Thus, we suggest that SOC play a pivotal role in governing the ability of hibernator hearts to maintain their function during the transition in and out of hibernating states.

Recording of calcium transient and analysis of calcium removal mechanisms in cardiac myocytes from rats and ground squirrels

With confocal microscopy, we recorded calcium transients and analyzed calcium removal rate at different temperatures in cardiac myocytes from the rat, a non-hibernator, and the ground squirrel, a hibernator. The results showed a remarkable increase of the diastolic level of calcium transients in the rat but no detectable change in the ground squirrel. Calcium transient of the ground squirrel, compared with that of the rat at the same temperature, had a shorter duration and showed a faster calcium removal. As indicated by the pharmacological effect of cyclopiazonic acid, calcium uptake by sarcoplasmic reticulum (SR) was the major mechanism of calcium removal, and was faster in the ground squirrel than in the rat. Our results confirmed the essential role of SR in hypothermia-tolerant adaptation, and negated the importance of Na-Ca exchange. We postulated the possibility to improve hypothermia-tolerance of the cardiac tissue of non-hibernating mammals.

Effects of cold cardioplegia on pH, Na, and Ca in newborn rabbit hearts

Many studies suggest myocardial ischemia-reperfusion (I/R) injury results largely from cytosolic proton (Hi)-stimulated increases in cytosolic Na (Nai), which cause Na/Ca exchange-mediated increases in cytosolic Ca concentration ([Ca]i). Because cold, crystalloid cardioplegia (CCC) limits [H]i, we tested the hypothesis that in newborn hearts, CCC diminishes Hi, Nai, and Cai accumulation during I/R to limit injury. NMR measured intracellular pH (pHi), Nai, [Ca]i, and ATP in isolated Langendorff-perfused newborn rabbit hearts. The control ischemia protocol was 30 min for baseline perfusion, 40 min for global ischemia, and 40 min for reperfusion, all at 37°C. CCC protocols were the same, except that ice-cold CCC was infused for 5 min before ischemia and heart temperature was lowered to 12°C during ischemia. Normal potassium CCC solution (NKCCC) was identical to the control perfusate, except for temperature; the high potassium (HKCCC) was identical to NKCCC, except that an additional 11 mmol/l KCl was substituted isosmotically for NaCl. NKCCC and HKCCC were not significantly different for any measurement. The following were different ( P < 0.05). End-ischemia pHi was higher in the CCC than in the control group. Similarly, CCC limited increases in Nai during I/R. End-ischemia Nai values (in meq/kg dry wt) were 115 ± 16 in the control group, 49 ± 13 in the NKCCC group, and 37 ± 12 in the HKCCC group. CCC also improved [Ca]i recovery during reperfusion. After 40 min of reperfusion, [Ca]i values (in nmol/l) were 302 ± 50 in the control group, 145 ± 13 in the NKCCC group, and 182 ± 19 in the HKCCC group. CCC limited ATP depletion during ischemia and improved recovery of ATP and left ventricular developed pressure and decreased creatine kinase release during reperfusion. Surprisingly, CCC did not significantly limit [Ca]i during ischemia. The latter is explained as the result of Ca release from intracellular buffers on cooling.

Adaptive mechanisms of intracellular calcium homeostasis in mammalian hibernators

Intracellular Ca2+ homeostasis is a prerequisite for a healthy cell life. While cells from some mammals may suffer dysregulation of intracellular Ca2+ levels under certain deleterious and stressful conditions, including hypothermia and ischemia, cells from mammalian hibernators exhibit a remarkable ability to maintain a homeostatic intracellular Ca2+ environment. Compared with cells from non-hibernators, hibernator cells are characterized by downregulation of the activity of Ca2+ channels in the cell membrane, which helps to prevent excessive Ca2+ entry. Concomitantly, sequestration of Ca2+ by intracellular Ca2+ stores, especially the sarcoplasmic/endoplasmic reticulum, is enhanced to keep the resting levels of intracellular Ca2+ stable. An increase in stored Ca2+ in heart cells during hibernation ensures that the levels of Ca2+messenger are sufficient for forceful cell contraction under conditions of hypothermia. Maintenance of Na+ gradients, viaNa+—Ca2+ exchangers, is also important in the Ca2+ homeostasis of hibernator cells. Understanding the adaptive mechanisms of Ca2+ regulation in hibernating mammals may suggest new strategies to protect nonhibernator cells, including those of humans, from Ca2+-induced dysfunction.

Contractile reserve and intracellular calcium regulation in mouse myocytes from normal and hypertrophied failing hearts

Mouse myocyte contractility and the changes induced by pressure overload are not fully understood. We studied contractile reserve in isolated left ventricular myocytes from mice with ascending aortic stenosis (AS) during compensatory hypertrophy (4-week AS) and the later stage of early failure (7-week AS) and from control mice. Myocyte contraction and [Ca(2+)](i) transients with fluo-3 were measured simultaneously. At baseline (0.5 Hz, 1.5 mmol/L [Ca(2+)](o), 25 degrees C), the amplitude of myocyte shortening and peak-systolic [Ca(2+)](i) in 7-week AS were not different from those of controls, whereas contraction, relaxation, and the decline of [Ca(2+)](i) transients were slower. In response to the challenge of high [Ca(2+)](o), fractional cell shortening was severely depressed with reduced peak-systolic [Ca(2+)](i) in 7-week AS compared with controls. In response to rapid pacing stimulation, cell shortening and peak-systolic [Ca(2+)](i) increased in controls, but this response was depressed in 7-week AS. In contrast, the responses to both challenge with high [Ca(2+)](o) and rapid pacing in 4-week AS were similar to those of controls. Although protein levels of Na(+)-Ca(2+) exchanger were increased in both 4-week and 7-week AS, the ratio of SR Ca(2+)-ATPase to phospholamban protein levels was depressed in 7-week AS compared with controls but not in 4-week AS. This was associated with an impaired capacity to increase sarcoplasmic reticulum Ca(2+) load during high work states in 7-week AS myocytes. In hypertrophied failing mouse myocytes, depressed contractile reserve is related to an impaired augmentation of systolic [Ca(2+)](i) and SR Ca(2+) load and simulates findings in human failing myocytes.

Subcellular Ca2+ distribution with varying Ca2+ load in neonatal cardiac cell culture

Recent work in our laboratory has investigated and modeled subcellular calcium compartmentation and Ca2+ movement under steady-state control conditions. This experimental study is directed to the further description and quantitation of cellular calcium compartmentation patterns and movements as correlated with contraction in neonatal rat cardiac myocytes in culture under a variety of calcium loading conditions. Compartmental contents were assessed after incubations in various [Ca2+]o, 0 Na+/1 mM Ca2+, and 10 microM ouabain/1.0 mM Ca2+ test solutions. The cellular components investigated include sarcolemmal bound, sarcoplasmic reticulum (SR), and mitochondrial calcium. The results indicate that 1) sarcolemmal calcium binding is insensitive to changes in [Ca2+]o in the range tested (0.25-6.0 mM) while highly sensitive to changes in [Na+]i; 2) SR is sensitive to both changes in [Ca2+]o and [Na+]i and exhibits a maximum loading capacity of approximately 750 micromol Ca2+/kg dw; 3) in the [Ca2+]o range between 0.25 and 2.0 mM, contractile amplitude is proportional to SR content; 4) the mitochondria comprise a high-capacity calcium-containing compartment that is sensitive to changes in [Ca2+]o but does not reach saturation under the conditions tested (0.25-8.0 mM [Ca2+]o); 5) SR calcium is divided into at least two functionally discrete pools, one of which is available for release to the myofilaments during a normal ICa-triggered contraction and other of which is caffeine releasable but unavailable for release to the myofilaments during a normal triggered release; and 6) mitochondrial calcium serves as a reservoir of calcium capable of replenishing and/or augmenting SR stores with anywhere from 10% to 50% of mitochondrial calcium cycling through SR calcium compartments.