Ca2+ uptake in the sarcoplasmic reticulum from the systemic heart of octopod cephalopods

We have measured Ca2+ uptake in crude homogenates of heart tissue, as well as cell shortening and ionic currents in isolated myocytes exposed to caffe ine, to characterize Ca2+ uptake in the sarcoplasmic reticulum (SR) of the systemic heart of octopus. The maximal rate of SR Ca2+ uptake in crude homo genates of octopus heart was 43+/-4 (mean +/- S.E.M., N=7), compared with 2 8+/-2 nmol min(-1) mg(-1) protein (N=4) in homogenates of rat heart. The Ca 2+-dependency of SR Ca2+ uptake was similar for the two species, with a Ca2 + 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) ca ffeine resulted in cell shortening to 53+/-2% of the resting cell length an d an inward trans-sarcolemmal ionic current, The charge carried by this cur rent was 3.28+/-0.70 pC pF(-1) (mean +/- S.E.M., N=5) corresponding to extr usion of 34.0+/-0.7 amol Ca2+ pF(-1) from the cell by Na+/Ca2+ exchange. Th is is approximately 50 times more than the Ca2+ carried by the Ca2+ current elicited by a 200 ms depolarization from -80 to 0 mV and corresponds to an increase in the total intracellular [Ca2+] of 404+/-86 mu mol l(-1) non-mi tochondrial volume due to Ca2+ release from the SR. Thus, we find that at 2 0 degrees C in the SR both Ca2+ content and Ca2+ uptake rate in the systemi c heart of octopus are comparable with or larger than the corresponding val ues 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 syste mic heart of cephalopods.