Contribution of sarcolemmal calcium current to total cellular calcium in postnatally developing rat heart.

OBJECTIVE: The aim was to determine the contribution of sarcolemmal Ca influx through L-type Ca current to total cellular Ca in newborn and postnatally developing rat heart.
METHODS: Whole cell voltage clamp was used to study L-type Ca current in freshly isolated ventricular cells of 1, 6, 10, 15, 30-day-old and adult (120-150 days) rats. Amplitude, current density, inactivation rate and time integral of Ca current were determined at the experimental temperature of 36 degrees C. Width, length, surface area, volume and surface-to-volume ratio of the isolated cells were also determined. Using the time integral of Ca current and volume of the myocytes the increment in total cellular Ca was calculated for the six developmental stages.
RESULTS: The amplitude of Ca current increased strongly during the postnatal maturation from 277 +/- 25 pA at day 1 to 1961 +/- 98 pA in the adult rat (P < 0.001). This increase was due to an almost proportional increase in cell size; accordingly the density of Ca current remained fairly constant, being 16.9 +/- 1.8, 17.0 +/- 1.8, 22.8 +/- 1.4, 21.2 +/- 1.5, 16.2 +/- 2.0 and 15.9 +/- 0.7 pA/pF for 1, 6, 10, 15, 30-day-old and adult rats, respectively. Charge transfer by Cd-sensitive Ca current during 200 ms voltage pulse from -45 mV to 0 mV increased from 7.18 +/- 1.00 pC at day one to 24.80 +/- 1.80 pC in the adult rat heart (P < 0.001). When normalized to the capacitive surface area of the myocytes the charge transfer by L-type Ca current was more than double in newborns (0.429 +/- 0.074 pC/pF) as compared to the adults (0.188 +/- 0.016 pC/pF) (P < 0.01). The difference is explained by slower inactivation rate of Ca current in newborn than adult rats. Time constant of the fast component was 5.92 +/- 0.62 ms and 4.5 +/- 0.4 ms (P < 0.05) for 1-day-old and adult rat, respectively. Time constant of the slow component decreased from 27.7 +/- 2.0 to 21.7 +/- 3.0, although the difference was not statistically significant (P = 0.26). The increment in total cellular Ca due to Ca influx through Ca channels was 54 +/- 9 mumol l-1 in 1-day-old rat and decreased steadily during postnatal maturation to 8 +/- 1 mumol l-1 in the adult rat (P < 0.001). The change is partly due to faster inactivation of Ca current in adults but mainly as a consequence of decreasing surface-to-volume ratio of growing myocytes. Sarcolemmal surface area increases almost 9 times from 1.001 x 10(3) microns2 at day 1 to 8.675 x 10(3) microns2 for the adult rat, whereas increase in cell volume is about 28-fold from 0.991 x 10(3) to 27.74 x 10(3) microns3: accordingly surface-to-volume ratio decreases from 1.05 +/- 0.02 at day 1 to 0.36 +/- 0.01 in the adult rat (P < 0.01).
CONCLUSIONS: Contribution of sarcolemmal Ca influx through L-type Ca channels is over 6 times higher in newborn than mature rats. Therefore, the strong dependence of neonatal rat heart on extracellular Ca for contractile activation is, at least partially, explained by greater contribution of L-type Ca current to intracellular Ca.