Calcium Homeostasis In Cardiomyocytes Research Articles

Intracellular calcium levels are unchanged in the diabetic heart.

The quality control indices of myocyte isolation (viability, yield, survival time, cell response, etc.) suggest that the adult rat myocyte model is stable and useful in [Ca2+]i measurements and functional studies at the cellular level. Moreover, diabetic cardiomyocytes are a valuable model for studying cellular functions of the diabetic heart as they retain most of the features of cardiac dysfunction of intact rat. Data from our studies indicate that the basal [Ca2+]i in both quiescent and electrically-stimulated cells is not changed. Thus, resting levels of [Ca2+]i and basal [Ca2+]i transients may not reflect the abnormalities observed in diabetes until the system is challenged by certain stimuli. [Ca2+]i responses to isoproterenol are depressed in both resting and stimulated diabetic cells. This suggests an alteration in the beta-adrenergic pathway, possibly related to the beta-adrenoceptor deficiency reported in the diabetic heart. SR Ca-ATPase is also involved in the isoproterenol-induced [Ca2+]i changes. Moreover, the decreased maximum response to 8-bromo-cAMP provides evidence of a post-receptor alteration in the pathway. Diabetic myocytes are more sensitive to ouabain, whereas the maximum response to ouabain was depressed. This may be the result of depressed Na-K ATPase and increased [Na+]i. In diabetic myocytes, rapid cooling contractures and caffeine contractures are depressed, whereas caffeine-induced Ca2+ transients are decreased. Ryanodine binding suggests a decreased number of high-affinity binding sites in the SR of diabetic myocytes. Additionally, there are indications that SR releasable calcium is reduced and that the major functions of SR, notably uptake, release and storage, may be depressed in diabetic myocytes. Finally, L-type Ca(2+)-channels are quantitatively and qualitatively altered in diabetes. Insulin treatment normalizes most of the diabetes-induced changes in cardiomyocytes, suggesting that metabolic alterations due to insulin deficiency play an important role in diabetic cardiomyopathy. Results from several studies show that in diabetes the function of major organelles which handle [Ca2+]i in myocytes is depressed, which in turn causes the alteration of [Ca2+]i mobilization in myocytes. Different second messenger systems involved in E-C coupling may also be altered due to the metabolic impairments. The rapid increase in our understanding of the pathophysiology of calcium homeostasis in cardiomyocytes will be forthcoming as the powerful new tools of molecular and structural biology are used to investigate the regulation of the Ca2+ transport system.

Relationship between Energetic, Ionic, and Functional Status in the Perfused Rat Heart Following Thermal Injury: A31P and23Na NMR Study

To test the hypothesis that energy deficits and intracellular ion derangements may be the cellular basis for intrinsic myocardial dysfunction in rats after burn trauma, we examined ATP metabolism, intracellular pH, sodium, and mechanical performance simultaneously in perfused beating hearts from sham burn or burned rats (43% TBSA, 3° scald burn, resuscitated for 24 hr with lactated Ringer's solution, Parkland formula). Intracellular calcium was also measured in myocytes harvested from parallel groups of sham burn and burn resuscitated rats. Burn trauma caused a 46% decrease in left ventricular developed pressure, a 69% decrease in +dP/dtmax, and a 72% decrease in −dP/dtmax. Intracellular to external standard sodium ratio increased (+58%) from 0.318 ± 0.027 to 0.500 ± 0.048 (P< 0.05), and intracellular calcium increased (+67%) from 206 ± 13 to 445 ± 37 nM(P< 0.01). Burn hearts exhibited decreased functional response to isoproterenol challenge compared to sham burn controls, but energy metabolism was similar in all hearts, regardless of burn injury. Our data suggest that burn trauma alters intracellular cardiomyocyte calcium and sodium homeostasis, and ionic derangements are not related to either altered intracellular pH or high energy phosphate deficits.

Calcium homeostasis in cardiomyocytes isolated from heat-shocked rats.

The cellular mechanism of heat shock-mediated cardioprotection is still under debate. Because heat pretreatment negatively affects the normoxic left ventricular contractile performance in vitro when the extracellular Ca2+ concentration ([Ca2+]o) is relatively low (0.65-1.25 mM), the intracellular Ca2+ homeostasis was studied in more detail in cardiomyocytes isolated from adult rats 24 h after heat stress (42 degrees C for 15 min) or anesthesia (control). Sensitivity to Ca2+ overload was assessed by exposure to veratridine (quiescent cells) or to [Ca2+]o ranging from 0.125 to 20 mM in quiescent and paced cardiomyocytes. The fraction of irreversibly hypercontracted cells was not different between groups. The fura-2 fluorescence ratio (I340/I380), which was used as a measure for cytoplasmic Ca2+ concentration ([Ca2+]i) in quiescent cells after exposure to [Ca2+]o (0.5-10 mM), was also not different between groups. Myofilament Ca2+ sensitivity was assessed in paced (0.5 Hz) cells by simultaneous measurement of [Ca2+]i transients and cell shortening. At stepwise increases of [Ca2+]o from 1 to 10 mM, these parameters were comparable between groups. The diastolic cell length shortened progressively and equally in both groups after increasing [Ca2+]o. However, within 2 min of return from 10 to 1 mM [Ca2+]o, cells from heat-shocked rats retained the same length, whereas cells from control rats contracted further (P = 0.05). These data suggest that heat stress improves relaxation after challenge with high [Ca2+]o.

766-5 Depressed Intracellular Calcium Homeostasis and Mechanical Properties of Cardiomyocytes in Annexin VI Transgenic Mouse

Annexins are a unique family of calcium-dependent, phospholipid-binding proteins found in various tissues. Annexin VI, a major member in the family, exists in the heart and acts as a potent regulator of the sarcoplasmic reticulum calcium-release channel, cardiac L-type calcium channel and Na + /Ca 2+ exchanger. To investigate the role of the regUlation of annexin VI in intracellular calcium homeostasis and mechanical properties of cardiomyocytes. we used the transgenic mice which overexpresses annexin VI (TG, n = 6) and control mice (C, n = 6) at three months of age. A 10-fold increase in annexin VI was revealed by Western blotting in TG mice compared to the controls. No differences were found between TG and C groups in heart/body wt., lung/body wt. or myocyte size. Cardiomyocytes from TG and C mice were isolated and field paced at 0.25, 0.5 and 1.0 Hz (15,30 and 60 beats per minute). The myocytes of TG mice demonstrated significant decreases in shortening fraction (% shortening, TG vs C, 4.59 ± 0.42 vs 8.62 ± 0.85, P &lt; 0.01), rate of shortening (+dl/dt, μ m/sec, TG vs C, 93.54 ± 24.31 vs 148.77 ± 20.23, P &lt; 0.05) and rate of relengthening (-dl/dt. μ m/sec, TG vs C, 59.28 ± B.32 vs 119.66 ± 15.94. P &lt; 0.01) at 1.0, but not at 0.25 or 0.5 Hz stimulation. FURA-2 loaded myocytes from TG mice showed lower baseline levels of calcium at all three stimulation frequencies respectively (340/380 ratio, TG vs C, 0.526 ± 0.075 vs 0.968 ± 0.550 at 0.25 Hz, P &lt; 0.001; 0.566 ± 0.044 vs 0.9B2 ± 0.050 at 0.5 Hz, P &lt; 0.001; 0.564 ± 0.043 vs 1.016 ± 0.051 at 1.0 Hz, P &lt; 0.001). The amplitude of calcium signals decreased in annexin VI TG mice at 0.25 Hz (340/380 ratio. TG vs C, 0.974 ± 0.14B vs 0.541 ± 0.082, P &lt; 0.05) and at 0.5 Hz stimulation (0.858 ± 0.141 vs 0.503 ± 0.077, P &lt; 0.05). The duration of calcium signals was attenuated in TG mice compared to control at 0.25 Hz (sec, 1.033 ± 0.227 vs 1.507 ± 0.150, P &lt; 0.05) We conclude that overexpression of annexin VI in mouse myocytes causes depression in the intracellular free calcium concentration. and frequencydependent depression in the extent and rate of the shortening and relengthening.