Changes in [Ca2+]i, [Na+]i and Ca2+ current in isolated rat ventricular myocytes following an increase in cell length

Isolated rat ventricular myocytes were stretched using carbon fibres to investigate the mechanisms underlying the increase in contraction following stretch. 2. [Ca2+]i and [Na+]i were monitored using the fluorescent indicators fura-2 and sodium-binding benzofuran isophthalate, respectively. The L-type Ca2+ current was recorded simultaneously with contraction using the perforated patch-clamp technique. 3. Mechanical stretch caused an immediate increase in contraction, followed by a slow increase. Contraction was prolonged immediately after the stretch, but did not change during the slow phase. 4. The Ca2+ transient did not change immediately after the stretch. The slow increase in contraction was accompanied by an increase in the amplitude of the Ca2+ transient. However, diastolic [Ca2+]i did not change significantly following stretch. 5. [Na+]i did not change significantly either immediately, or during the slow increase in contraction, after the stretch. 6. The L-type Ca2+ current was not significantly altered either by mechanical loading of the cell with carbon fibres or by stretching the cell. 7. These results suggest that: (1) the rapid increase in contraction following a stretch is due to an increase in myofilament Ca2+ sensitivity rather than to changes in the L-type Ca2+ current or [Na+]i; and (2) a slow increase in the Ca2+ transient underlies the slow increase in contraction in isolated myocytes, but is not caused by either an increase in diastolic [Ca2+]i or a change in [Na+]i (and hence Ca2+ influx via Na(+)-Ca2+ exchange) or a change in myofilament Ca2+ sensitivity.