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1 Faculty of Life Sciences, The Univeristy of Manchester, Manchester, United Kingdom; School of Biosciences, The University of Birmingham, Birmingham, United Kingdom
2 School of Biosciences, The University of Birmingham, Birmingham, United Kingdom
3 Faculty of Life Sciences, The Univeristy of Manchester, Manchester, United Kingdom
* To whom correspondence should be addressed. E-mail: ginaljgalli{at}hotmail.com.
The relative contributions of the sarcoplasmic reticulum (SR), the L-type Ca2+ channel and the Na+/Ca2+ exchanger (NCX) were assessed in turtle ventricular myocytes using epi-fluorescent microscopy and electrophysiology. Confocal microscopy images of turtle myocytes revealed spindle shaped cells which lack T-tubules and have 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. Depolarising 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-1, which lead to an overall total Ca2+ influx of 64.1 ± 9.3 µM L-1. 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 depolarising voltage steps to 0 mV was 58.5 ± 7.7 µmol L-1 and 26.7 ± 3.2 µmol L-1 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.
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