Calcium flux in turtle ventricular myocytes.
The relative contribution of the sarcoplasmic reticulum (SR), the L-type Ca(2+) channel and the Na(+)/Ca(2+) 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 Ca(2+)-sensitive dye Fura-2 elicited Ca(2+) 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 Ca(2+) 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 Ca(2+) currents originating from L-type Ca(2+) channels (I(Ca)). The density of I(Ca) was 3.2 +/- 0.5 pA/pF, which led to an overall total Ca(2+) influx of 64.1 +/- 9.3 microM/l. NCX activity was measured as the Ni(+)-sensitive current at two concentrations of intracellular Na(+) (7 and 14 mM). Total Ca(2+) influx through the NCX during depolarizing voltage steps to 0 mV was 58.5 +/- 7.7 micromol/l and 26.7 +/- 3.2 micromol/l at 14 and 7 mM intracellular Na(+), respectively. In the absence of the SR and L-type Ca(2+) channels, the NCX is able to support myocyte contraction independently. Our results indicate turtle ventricular myocytes are primed for sarcolemmal Ca(2+) transport, and most of the Ca(2+) used for contraction originates from the L-type Ca(2+) channel.
Galli
Gina L J
GL
Faculty of Life Sciemces, The University of Manchester, Core Technology Facility, 46 Grafton Street, Manchester, M13 9NT UK. ginaljgalli@hotmail.com
Taylor
Edwin W
EW
Shiels
Holly A
HA
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology*; D002478:Cells, Cultured; D006352:Heart Ventricles / Q000166:cytology; D015640:Ion Channel Gating / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology; D032383:Myocytes, Cardiac / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000502:physiology*; D014426:Turtles / Q000502:physiology*; D016276:Ventricular Function
D020746:Calcium Channels, L-Type; D002118:Calcium