Ca(2+) uptake in the sarcoplasmic reticulum from the systemic heart of octopod cephalopods

We have measured Ca(2+) uptake in crude homogenates of heart tissue, as well as cell shortening and ionic currents in isolated myocytes exposed to caffeine, to characterize Ca(2+) uptake in the sarcoplasmic reticulum (SR) of the systemic heart of octopus. The maximal rate of SR Ca(2+) uptake in crude homogenates of octopus heart was 43+/-4 (mean +/- s.e.m., N=7), compared with 28+/-2 nmol min(-)(1 )mg(-)(1) protein (N=4) in homogenates of rat heart. The Ca(2+)-dependency of SR Ca(2+) uptake was similar for the two species, with a Ca(2+) activity at half-maximal uptake rate (pCa(50)) of 6.04+/-0.02 for octopus and 6.02+/-0.05 for rat. Exposure of isolated myocytes to 10 mmol l(-)(1) caffeine resulted in cell shortening to 53+/-2 % of the resting cell length and an inward trans-sarcolemmal ionic current. The charge carried by this current was 3.28+/-0.70 pC pF(-)(1) (mean +/- s.e.m., N=5) corresponding to extrusion of 34.0+/-0.7 amol Ca(2+ )pF(-)(1) from the cell by Na(+)/Ca(2+) exchange. This is approximately 50 times more than the Ca(2+) carried by the Ca(2+) current elicited by a 200 ms depolarization from -80 to 0 mV and corresponds to an increase in the total intracellular [Ca(2+)] of 404+/-86 (&mgr;)mol l(-)(1) non-mitochondrial volume due to Ca(2+) release from the SR. Thus, we find that at 20 degrees C in the SR both Ca(2+) content and Ca(2+) uptake rate in the systemic heart of octopus are comparable with or larger than the corresponding values obtained in the rat heart. These results support the argument that the SR may play an important role in the regulation of contraction in the systemic heart of cephalopods.