Calcium flux in turtle ventricular myocytes.

Click for Abstract : The relative contribution of the sarcoplasmic reticulum (SR), the L-type Ca2+�channel and the Na+/Ca2+�exchanger (NCX) were assessed in turtle ventricular myocytes using epifluorescent microscopy and electrophysiology. Confocal microscopy images of turtle myocytes revealed spindle-shaped cells, which lacked T-tubules and had a large surface area-to-volume ratio. Myocytes loaded with the fluorescent Ca2+-sensitive dye Fura-2 elicited Ca2+�transients, which were insensitive to ryanodine and thapsigargin, indicating the SR plays a small role in the regulation of contraction and relaxation in the turtle ventricle. Sarcolemmal Ca2+�currents were measured using the perforated-patch voltage-clamp technique. Depolarizing voltage steps to 0 mV elicited an inward current that could be blocked by nifedipine, indicating the presence of Ca2+�currents originating from L-type Ca2+�channels (ICa). The density of ICa�was 3.2 � 0.5 pA/pF, which led to an overall total Ca2+�influx of 64.1 � 9.3 ?M/l. NCX activity was measured as the Ni+-sensitive current at two concentrations of intracellular Na+�(7 and 14 mM). Total Ca2+�influx through the NCX during depolarizing voltage steps to 0 mV was 58.5 � 7.7 ?mol/l and 26.7 � 3.2 ?mol/l at 14 and 7 mM intracellular Na+, respectively. In the absence of the SR and L-type Ca2+�channels, the NCX is able to support myocyte contraction independently. Our results indicate turtle ventricular myocytes are primed for sarcolemmal Ca2+�transport, and most of the Ca2+�used for contraction originates from the L-type Ca2+�channel.