[The effect of alpha 1-adrenergic stimulation of the myocardium depends on the opposite actions of 2 receptor subtypes]

Two different myocardial alpha 1-adrenergic receptor subtypes have been recently described. Since their functional role is still unknown we examined the effect of alpha 1A and alpha 1B on contractile properties, cell Ca2+ homeostasis and myofilament sensitivity to Ca2+ in indo-1 loaded ventricular myocytes. Non selective alpha 1-adrenergic stimulation increases twitch amplitude, systolic Ca2+ and myofilament sensitivity. Same effects but amplified are associated with alpha 1A-receptors stimulation. In contrast, alpha 1B-receptors stimulation decreases twitch amplitude and systolic Ca2+ below control value. Myocardial alpha 1-adrenergic stimulation results from the opposite actions of the 2 subtypes.