Insulin is an important metabolic hormone that influences ion transport mechanisms and the inotropic state of the heart (Hiraoka, 2003). At room temperature insulin has been reported to increase L-type calcium current (I_Ca,L) in human atrial and rat ventricular isolated myocytes (Aulbach et al. 1999; Maier et al. 1999) and this effect could contribute to its influence on cardiac contractility. However, we have recently shown that the effect of insulin on I_Ca,L may in fact vary with temperature: a stimulatory effect is observed at room temperature and an inhibitory one at 37 °C (Pabbathi et al. 2002). The present study was designed to see whether the inotropic effect of insulin is observed in myocytes under experimental conditions expected to result in an inhibitory effect on I_Ca,L.

Adult male guinea-pigs were killed by cervical dislocation and ventricular myocytes were isolated by a combination of mechanical and enzymatic dispersion. Ionic current measurements were made using the whole-cell patch clamp technique and cell contraction was assessed videometrically by measuring unloaded cell-shortening. Experiments were performed at physiological temperature (37°C).

Under conditions selective for I_Ca,L (Cs⁺-based pipette dialysate), peak I_Ca,L at a test potential of +10 mV was observed to be smaller in cells pre-incubated in 1 µM insulin than in insulin-free controls. Dual measurements of membrane current and unloaded shortening were then made (using a K⁺-based pipette dialysate). Lidocaine at 200 µM was present in the external solution to block residual Na current, and peak I_Ca,L and cell shortening were measured during a voltage step from -40 mV to +10 mV. In control cells, I_Ca,L amplitude was -2.52 ± 0.4 nA (n = 6; mean ± S.E.M.) and cells contracted by 6.37 ± 0.87 % of resting cell length (mean ± S.E.M.). In cells pre-incubated in 1 µM insulin and continuously superfused with insulin-containing solution during recording, I_Ca,L amplitude was -1.30 ± 0.26 nA (n = 6; P < 0.05 compared to control, Student’s unpaired t test), whilst cells contracted to 10.25 ± 1.05 % of resting cell length (P < 0.05 compared to control).

This work suggests that under conditions in which insulin decreases I_Ca,L it can nevertheless increase cell shortening.

Support by the National Heart Research Fund is gratefully acknowledged