Contractile responses of isolated adult rat and rabbit cardiac myocytes to isoproterenol and calcium.

Myocytes were isolated by Langendorff perfusion of rat or rabbit hearts with low calcium solution followed by collagenase and hyaluronidase, or by incubation of chunks of rat ventricular tissue in similar media. Cells were then placed in a bath on a microscope stage, superfused and electrically stimulated. Contraction amplitude and rate of change of length during contraction were measured using a video camera and edge detection monitor. Cells were selected for study using a number of criteria developed to identify and define a cell population able to give consistent inotropic responses over a long period. The maximum contraction amplitude with isoproterenol in rabbit cells was 0.244 micron (sarcomere length change) or 13.1% (percentage change in cell length), and the EC50 was 12.8 nM. The maximum contraction amplitude with isoproterenol did not differ significantly between rat and rabbit, between cells prepared by perfusion and those made from chunks, or when determined from non-cumulative rather than cumulative curves. The EC50 for isoproterenol in rat cells made by the perfusion method (cumulative curves) was 3.81 nM, significantly lower than in rabbit. The maximum amplitude obtained with increasing concentrations of calcium was not significantly different from that with isoproterenol under any condition. The EC50 for calcium averaged 2.78 mM in rat cells made by the perfusion method (cumulative curves) and was significantly greater than that in rabbit (1.4 mM). Maximum rates of contraction for rat cells averaged 4.59 micron/s in 8 mM calcium. Rat cells contracted faster than they relaxed, whereas rabbit cells in 8 mM calcium relaxed faster than they contracted. Rat cells, maximally activated by either calcium or isoproterenol, contracted significantly faster than rabbit. There was no difference in rates of contraction (or relaxation) between rat cells prepared by perfusion and those made from chunks of tissue.