Involvement of receptor subtypes in the alpha 1-adrenoceptor mediated activation of Na/K/2Cl-cotransport and K+ channels was studied in isolated perfused spontaneously beating rat hearts stimulated by phenylephrine (30 mumol/l) in the presence of a beta-adrenoceptor antagonist (1 mumol/l timolol). The effects of alpha 1-adrenoceptor stimulation on K+ translocation mechanisms were studied by measuring the efflux of 86Rb+ (a potassium analogue). The effects of 50 mumol/l bumetanide (Na/K/2Cl-cotransport inhibitor) and 0.1-0.3 mmol/l 4-aminopyridine (inhibitor of K+ channels) were studied in the presence of alpha 1-adrenoceptor subtype selective antagonists. Bumetanide reduced the alpha 1-adrenoceptor mediated increase in 86Rb+ efflux by 53 +/- 16.4% (n = 14, P < 0.001) in hearts pretreated with the preferentially alpha 1B-adrenoceptor antagonist chloroethylclonidine (CEC, 10 mumol/l), and by 35 +/- 7.3% (n = 15, P < 0.001) in the presence of the preferentially alpha 1D-adrenoceptor antagonist BMY 7378 (1 mumol/l). In the presence of the preferentially alpha 1A-adrenoceptor antagonist 5-methylurapidil (10 mumol/l), however, bumetanide had no effect on the response to phenylephrine. 4-Aminopyridine reduced the phenylephrine stimulated 86Rb+ efflux in the presence of 5-methylurapidil, but the effect of the K(+)-channel blocker was eliminated in CEC treated hearts. Thus the effects of the two translocation inhibitors were influenced differently by the two subtype selective antagonists, showing that alpha 1-adrenoceptor stimulation activates a bumetanide sensitive Na/K/2Cl-cotransport mechanism in the rat heart mainly through the alpha 1A-receptor subtype while the 4-aminopyridine sensitive K+ channels, are mainly activated by the alpha 1B-adrenoceptor subtype.