Possible reasons for the variability of the inotropic insulin effect in papillary muscles of ground squirrel myocardium

The effects of insulin (0.1-50 nM) on isometric twitch force (0.1 to 1.0 Hz; 30 +/- 1 degree C; 1.8 mM Ca(2+)) were studied in right ventricular papillary muscles from active ground squirrels of different seasons (summer, n = 14; autumn, n = 16 and winter interbout, n = 16) in control conditions and after one-hour pretreatment of PM with 2 mkM nifedipine (an L-type Ca(2+)-channel inhibitor) and 1.0 mM orthovanadate (a tyrosine phosphatase inhibitor). In active animals of different seasonal periods insulin causes both positive and negative inotropic effects. At low frequencies (0.1-0.5 Hz), insulin of low concentrations (0.1-1.0 nM) induces a transient (within the first 20 min after application) positive effect (about 15-25%). Application of high hormone concentration (10 nM) in a low range of stimulation frequencies causes a biphasic effect (a small initial positive inotropic effect followed by a marked negative one). At frequencies above 0.5-Hz stimulation, insulin of 10 nM concentration causes presumably a negative inotropic effect. It was proposed that ICaL is possibly involved in the insulin-induced negative inotropy in ground squirrels hearts. Alteration of protein phosphorylation in tyrosine residues is known to be a major link in the mechanism of insulin action. We performed a study on orthovanadate action (a known inhibitor of tyrosine phosphatase) on the inotropic insulin effect. In the group of summer animals the pretreatment of papillary muscles with orthovanadate (100 mkM) does not change the negative inotropic effect of insulin in a low range of stimulation frequencies but almost completely removes this effect at stimulation frequencies above 0.3 Hz (n = 4). Nifedipine (1-1.5 hr pretreatment), a blocker of L-type calcium channel, reduces the inhibitory effect of insulin in autumn and winter animals, and on the contrary intensifies it in summer animals. This fact indicates that different mechanisms must be involved in insulin actions in animals of summer and winter periods. The main findings of the present study are that insulin induces positive, negative or no inotropic effects in papillary muscles of ground squirrels myocardium. The character of the effects of insulin depends on the physiological state of animals; time and concentrations of the hormone applied; affected by conditions that alter cellular Ca(2+) loading and the ratio of protein-tyrosine kinases/phosphatases activity.