Ultrastructure and function of isolated myocytes after calcium depletion and repletion

We describe a system in which the contractile response of a single isolated Ca-tolerant rat myocyte can be monitored while stimulated in Ca-free (37 degrees C) and then Ca-containing medium. This same cell is then processed for ultrastructural analysis. We find unattached isolated Ca-tolerant myocytes can withstand Ca depletion and repletion. They respond to stimulation after Ca repletion with healthy twitches and are structurally intact. The cell surface of the isolated myocyte appears similar to that seen in the intact myocytes even after 15 min in Ca-free solution. This is in marked contrast to myocardial cells in tissue where Ca depletion causes separation of the layers of the glycocalyx. When the cell is attached to a fine pipette so that some of the sarcolemma is under stress, the sequence of Ca depletion and repletion drives the cell into hypercontracture and loss of structural integrity that is characteristic of the Ca paradox.