Contractile Arrest Increases Sarcoplasmic Reticulum Calcium Uptake and SERCA2 Gene Expression in Cultured Neonatal Rat Heart Cells

We developed protocols with intact cultured neonatal rat myocytes to directly evaluate the function of the sarcoplasmic reticulum (SR) Ca-ATPase (or SERCA2), Na-Ca exchange (Na-CaX), and slow Ca transport systems (mitochondria and sarcolemmal Ca-ATPase}. Spontaneously beating control cells were compared with cells cultured for 2 days in the presence of verapamil (verapamil-arrested cells, VA). Intracellular calcium (Cai) transients were measured by use of indo-1 during (1) spontaneous twitches, (2) contractures induced by rapid application of caffeine (CafC, with and without Nao), and (3) twitches induced by brief depolarizations with high [K], solution (K-twitches). We also measured mRNA levels for the SR Ca-ATPase and Na-CaX in the same experimental preparations. The tl/2 for [Ca]i decline when both the SR Ca uptake and Na-CaX were prevented was the same for control and VA cells (≈20 seconds), indicating unaltered slow Ca transport systems. Similarly, there was no significant difference in the t1/2 of CafC when Na-CaX was the main mechanism responsible for [Ca]i decline (t1/2 ≈1.5 seconds), indicating unaltered Na-CaX. Conversely, we found nearly a twofold increase in the rate of [Ca]i decline during K-twitches (control t1/2, 0.84±0.05 seconds; VA t1/2, 0.48±0.06 second; P