Electrical activity and contraction in cells isolated from rat and guinea-pig ventricular muscle: a comparative study.

1. Contraction in single ventricular muscle cells from rat and guinea-pig heart was measured using an optical technique, while at the same time either action potentials were recorded or transmembrane currents were measured under voltage-clamp conditions. 2. When the membrane was depolarized to 0 mV, there was a phasic and a tonic component of the contraction in guinea-pig cells, whereas in rat cells only the phasic component was obvious. In both species the depolarizations evoked the second inward current (Isi). 3. In rat cells, when the membrane potential during a depolarization was varied over the range -40 to +60 mV, the amplitude of contraction first increased to a peak at a potential close to 0 mV, and then declined as the membrane potential became more positive. In contrast, contraction in guinea-pig cells measured under similar conditions continued to increase as the depolarization was increased, and the tonic component of contraction became more obvious at more positive potentials. Contraction amplitude in guinea-pig cells could also be increased by increasing pulse duration under conditions where the tonic component of contraction was prominent. 4. Contraction during depolarization was suppressed by ryanodine in rat cells, whereas in guinea-pig cells contraction persisted, but with a modified time course. Ryanodine did inhibit spontaneous contractions of guinea-pig cells during exposure to low extracellular sodium. 5. Nifedipine suppressed Isi and phasic contraction in both rat and guinea-pig cells. In guinea-pig cells these effects developed contemporaneously, but in rat cells substantial reduction of Isi occurred before marked suppression of contraction. 6. In rat cells exposed to strontium in place of external calcium, inactivation of Isi was slowed and contraction was prolonged, with a slower time-to-peak and relaxation. The time course of the action potential was modified and ryanodine no longer inhibited contraction of rat cells in the presence of strontium. 7. It is concluded that the amplitude of contraction in rat and guinea-pig ventricular cells is determined by calcium both entering through the surface membrane and released from internal stores, and that under normal conditions the balance is towards release from stores in rat cells, and towards entry through the surface in guinea-pig cells.