Intracellular and external recording from frog ventricular fibers during hypertonic perfusion

Isolated frog hearts perfused with hypertonic solutions passed through three reversible stages: I) altered spontaneous waveforms; II) absence of spontaneous activity but responsive to stimulation; III) nonresponsive. The following patterns of action potentials were observed with intracellular recording during stage I: 1) the plateaus gradually disappeared leaving short-duration spikes. 2) These spikes often occurred as high-frequency bursts usually accompanied by sustained contractions. Quiescent cells were often located adjacent to active ones, and many fibers fired at frequencies lower than that of the ECG. 3) Often a ‘slow wave’ gradually differentiated from the plateau and resulted in separation of a fast initial spike from a later slow wave. These observations suggest fiber-to-fiber junctional transmission rather than conduction within a syncytium. Hypertonic solutions may: a) depress fiber-to-fiber transmission, b) alter the time courses of the Na and K conductances and c) directly depress the contractile mechanism.