When the electrical properties of single, isolated, rat ventricular cells at low pH were examined, the amplitude of the action potential and its duration were found to decrease at low pH (<5.5). In voltage clamp experiments, the low pH depressed IS and slowed the inactivation process, yet the outward currents reflecting the plateau range of the action potential were not affected. The I V curve of IS shifted to less negative potentials at low pH. Similar results were obtained in ISr and IBa at

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... w pH. The alteration of the action potential of single ventricular cells at low pH is attributed to the suppression of IS. The mechanism of this reduction of IS might be due to the presence of a surface potential capable of affecting gating and permeation mechanisms of f. channel or to the protonation of some acidic group that must be ionized for the 1 channel to function normally. 403 404 A. YATANI and M. GOTO not been studied quantitatively. The membrane currents in cardiac tissue has been hindered by complex, syncytical factors of heart preparations such as spacial inhomogeneity, a considerable series resistance because of intracellular clefts and nonuniformed distribution of an injected current (JOHNSON and LIEBERMAN,1971; ATTWELL and COHEN, 1977; NOBLE, 1979) . In addition, depletion and accumulation of ions in the restricted extracellular spaces often occur (MoRAD, 1980; COHEN and KLINE, 1982). Viable, isolated single cell preparations should reduce or eliminate such problems. In the present experiments, we used isolated single rat ventricular cells and studied the effect of low pH on the action potential and the membrane currents using the voltage clamp technique. We also investigated the effect of low pH on the slow inward currents carried by Sr2+ or Ba2+ METHODS Cell isolation. The enzymatic dispersion procedure we used was similar to that reported by POWELL et al. (1980) . Adult rats weighing approx. 200-250 g were used. The chest was opened and the heart was extracted quickly and washed with Krebs solution. Retrograde perfusion through the aorta was performed using a Langendorff apparatus at 37°C. The perfusion procedure was as follows; 4 min with nominally Ca-free Krebs solution containing 1 mg/ml bovine serum albumin which was essentially fatty acid free (Sigma); 20 min with enzyme medium containing 0.4 mg/ml collagenase type I (Sigma). Thereafter, the ventricle was cut into 1 mm slices. The tissue was incubated at 37°C for 12-15 min in enzyme solution containing an additional 10 mg/ml bovine serum albumin. Dispersed cells were filtered and stored at 4°C in "Kraftbruhe" or "Power soup" KLOCKNER, 1980, 1982). Cells were generally used after 2-6 hr. About 30-60 % of the cells remained rod-shaped and quiescent in the Ca-containing solution. The cells with a clear margin and striations were chosen. Only cells showing a stable action potential (resting potential ranged between -75 and -80 mV) were used . Electrophysiology. The experimental chamber (total volume 0.2 ml) was mounted on the stage of an inverted phase-contrast microscope. Cells were dropped into the chamber. After the cells settled on the glass bottom on the chamber, solutions were superfused through the chamber at a rate of 2 ml/min. To secure the complete solution exchange, at least a 10-20 volume of bath solution was exchanged with test solutions. The cells were stimulated by a constant current (1-2 nA, 2-5 msec in duration) through the recording microelectrode. The tip resistance of a microelectrode was 10-20 MSS when filled with 3 M KCI. Voltage-clamp experiments were carried out using a Dagan 8100 (Minneapolis, Minn.), a single electrode voltage clamp apparatus. This instrument switches rapidly (0.5-20 kHz) from voltage recording Japanese Journal of Physiology LOW pH IN CALCIUM CHANNEL 405