Objective: The aim of this study was to gain further insights into the consequences of insulin-dependent diabetes mellitus on cardiomyocyte calcium handling.
Methods: The effects of steady state and transient changes in stimulus frequency on the intracellular Ca2+ transient and cell shortening were examined in left ventricular cardiomyocytes isolated from the hearts of control and streptozotocin-induced diabetic rats.
Results: During steady state stimulation diabetic rat cardiomyocytes displayed a slower decay of the Ca2+ transient and longer times for maximum cell shortening and re-lengthening. At 1.5 mM extracellular [Ca2+], increasing stimulus frequency over the range 0.2-1.0 Hz led to an increase in resting and peak [Ca2+]i as well as the amplitude of the transient in both the control and diabetic groups. At frequencies greater than 0.4 Hz the amplitude of the transient was significantly depressed in diabetic rat cells and this was not normalized by increasing extracellular [Ca2+] to 2.5 mM. Recovery of sarcoplasmic reticulum (SR) Ca2+ release was measured from the time course of restitution of the intracellular Ca2+ transient. In both control and diabetic rat cardiomyocytes recovery of the transient occurred in two phases. In diabetic rat myocytes, the initial rapid phase of restitution at intervals <1 s was markedly slowed. The fraction of Ca2+ recirculating between the SR and the cytosol was estimated from the decline in amplitude of transients following post-rest potentiation. There was no difference in this fraction between control and diabetic rat cells either at 1.5 or 2.5 mM extracellular [Ca2+].
Conclusion: The blunted frequency response of diabetic rat cardiomyocytes at frequencies greater than 0.4 Hz is consistent with reduced SR Ca2+ uptake leading to reduced SR Ca2+ content and subsequent release. At stimulus intervals greater than 1 Hz this is likely to be exacerbated by slower recovery of SR Ca2+ release. Despite the evidence for depressed SR Ca2+ uptake, the relative amount of Ca2+ recirculating within diabetic rat cardiomyocytes remains unaltered. This is most likely due to an accompanying reduction in Ca2+ efflux from the cell due either to depressed Na+/Ca2+ exchanger activity, or an elevation in intracellular Na+ levels.