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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.

1. The fluorescent indicator Fura-2 has been used to monitor intracellular [Ca2+] (Ca2+i) in myocytes isolated from the ventricles of rat hearts. 2. The relationships between diastolic Ca2+i, systolic Ca2+i and the Ca2+ content of the sarcoplasmic reticulum (SR; assayed using caffeine) have been studied during changes of stimulation rate and bathing [Ca2+] (Ca2+o). 3. When stimulation rate was increased, there were increases in diastolic Ca2+i, systolic Ca2+i and the Ca2+ content of the SR. 4. The SR inhibitor ryanodine (1 mumol l-1) decreased the size of the Ca2+i transient, and abolished the increase of Ca2+i produced by caffeine (10 mmol l-1). In the presence of ryanodine, increasing stimulation rate increased diastolic Ca2+i but not systolic Ca2+i. 5. Increasing Ca2+o led to increases of diastolic Ca2+i, systolic Ca2+i and SR Ca2+ content similar to those observed during changes in stimulation rate. 6. Ryanodine altered the relationship between systolic and diastolic Ca2+i during changes of Ca2+o. 7. These results are consistent with a change of diastolic Ca2+i leading to an increase in the Ca2+ content of the SR, and hence an increase in the size of the Ca2+i transient during changes in stimulation rate and Ca2+o.