Calcium flux in turtle ventricular myocytes

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 μM/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 μmol/l and 26.7 ± 3.2 μmol/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.

Temperature-dependence of L-type Ca2+ current in ventricular cardiomyocytes of the Alaska blackfish (Dallia pectoralis)

“…A number of in vivo and in vitro studies have examined the effects of temperature and oxygen deprivation on the cardiovascular status and cardiac physiology of air-breathing fishes (Costa et al 2005; Costa et al 2009; Iversen et al 2013; McKenzie et al 2007), but none have investigated cardiac excitation-contraction coupling at the level of the isolated cardiomyocyte. Our measurements of cardiomyocyte morphology revealed that ventricular cardiomyocytes of Alaska blackfish are long and thin, much like those of other non-mammalian species (Galli et al 2006; Galli et al 2009; Vornanen et al 2002). However, cell size, capacitance and surface area were generally 15-50% less than reported for other ectothermic species (Galli et al 2006; Galli et al 2009; Tiitu and Vornanen 2002; Vornanen 1997, 1998).…”

“…Our measurements of cardiomyocyte morphology revealed that ventricular cardiomyocytes of Alaska blackfish are long and thin, much like those of other non-mammalian species (Galli et al 2006; Galli et al 2009; Vornanen et al 2002). However, cell size, capacitance and surface area were generally 15-50% less than reported for other ectothermic species (Galli et al 2006; Galli et al 2009; Tiitu and Vornanen 2002; Vornanen 1997, 1998). Overall, the surface-to-volume ratio (1.03 μm −1 ) was less than reported for rainbow trout, burbot and crucian carp (1.15-1.2 μm −1 ; Tiitu and Vornanen 2002; Vornanen 1997, 1998).…”

“…ND, Not determined. Species are: Squalus acanthias , spiny dogfish; Danio rerio , zebrafish; Dallia pectoralis , Alaska blackfish; Carassius carassius , crucian carp; Trachemys scripta , red-eared slider turtle; Scomber japonicas , Pacific mackerel; Oncorynchus mykiss , rainbow trout; Thunnus orientalis , Pacific bluefin tuna; Trachemys scripta scripta , yellow-bellied turtle; Varanus exanthematicus , Varanid lizard; Acipenser baerii , Siberian sturgeon; Lampetra fluviatilis , European lamprey; Lota lota , burbot; Perca fluviatilis , European perch. a (Maylie and Morad 1995) b (Brette et al 2008) c Acclimation temperature 21°C (Zhang et al 2011) d (Vornanen and Paajanen 2004) e Carassius carassius : acclimation temperature 17°C; Both Carassius carassius and Oncorynchus mykiss recorded at 22°C (Vornanen 1998) f (Stecyk et al 2007) g (Shiels et al 2004) h Acclimation temperature 14°C, recorded at 19°C (Shiels et al 2014) i (Galli et al 2006) j (Galli et al 2009) k (Haworth et al 2014) l (Haverinen et al 2014) m (Shiels et al 2006) …”

Ca2+cycling in cardiomyocytes from a high-performance reptile, the varanid lizard (Varanus exanthematicus)

“…In most ectotherms, the majority of Ca 2+ necessary for contraction is extracellular in origin, regardless of activity level (1, 10, 16, 24, 37, 45, 49). In fish and turtles, extracellular Ca 2+ crosses the sarcolemmal membrane primarily through the l -type Ca 2+ channel, although in some species the Na + /Ca 2+ exchanger (NCX) can also contribute significantly (15, 16, 25, 27, 46–48). Aside from directly contributing activator Ca 2+ , the l -type Ca 2+ channel and the NCX may also serve to trigger the release of Ca 2+ from the SR (26) and contribute to the plateau of the cardiac action potential (AP) (47).…”

“…We have previously shown intracellular Ca 2+ transients (Δ[Ca 2+ ] i ) from turtle ventricular myocytes under steady-state conditions at room temperature are insensitive to SR inhibition, and contraction is supported exclusively by sarcolemmal pathways (15). In the present study we investigate the hypothesis that these Ca 2+ cycling pathways are enhanced in varanid lizard ventricular cardiomyocytes, thereby contributing to this species' superior cardiovascular performance.…”