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The effect of a calmodulin inhibitor on intracellular [Ca2+] and contraction in isolated rat ventricular myocytes.

1. The effect of the calmodulin inhibitor N‐(6‐aminohexyl)‐5‐chloro‐1‐naphthalenesulphonamide (W7; 10 microM) on intracellular [Ca2+] ([Ca2+]i) and [H+], and on contraction, has been studied in myocytes isolated from the ventricles of rat hearts. [Ca2+]i and [H+] were monitored using the fluorescent dyes Fura‐2 and 2',7'‐bis(carboxyethyl)‐5,6‐carboxyfluorescein (BCECF) respectively. 2. W7 decreased the size of both the Fura‐2 fluorescence (a function of [Ca2+]i) transient and twitch, but had no effect on their time course. 3. The decrease in the size of the Fura‐2 fluorescence transient in the presence of W7 was accompanied by a decrease in the increase of Fura‐2 fluorescence that could be elicited by releasing Ca2+ from the sarcoplasmic reticulum using 10 mM‐caffeine. 4. There was a decrease in the apparent sensitivity of the contractile proteins to Ca2+ in the presence of W7 which may account, in part, for the decrease in the twitch observed in the presence of W7. 5. Test beats were interpolated at different test intervals after a train of steady‐state contractions. Mechanical restitution curves were constructed by plotting the size of the test beat against the test interval. Both the size and the duration of the twitch increased as the test interval was prolonged. W7 slowed this mechanical restitution but had no effect on the changes in the duration of the twitch. 6. Intracellular pH was not altered by W7. 7. These results are discussed in terms of the known actions of calmodulin and W7.