CHARACTERIZATION OF STIMULATION OF PHOSPHOINOSITIDE HYDROLYSIS BY ALPHA(1)-ADRENERGIC AGONISTS IN ADULT-RAT HEARTS

The coupling of the pharmacologically defined alpha(1A)- and alpha(1B) -adrenoceptors to the hydrolysis of phospho[H-3]inositides (PI) was in vestigated in ventricular myocytes freshly isolated from adult rat hea rts. The alpha(1)-adrenoceptor population in the heart was characteriz ed by competitive binding experiments using [H-3]prazosin and the alph a(1A)-adrenoceptor-selective antagonist 5-methyl urapidil. It was hete rogeneous with similar to 25% being pharmacologically of the alpha(1A) -adrenoceptor subtype and 75% being of the alpha(1B)-adrenoceptor subt ype. Epinephrine, norepinephrine, or phenylephrine stimulated PI hydro lysis in the presence or absence of propranolol. The greatest stimulat ion (7-fold) was with epinephrine. The half-maximum effective concentr ations for agonists were similar to 0.5-3.5 and 0.2 mu M in the absenc e and presence of propranolol, respectively. The inhibition by Ei-meth yl urapidil of the stimulation of PI hydrolysis by a fixed concentrati on of epinephrine fitted a two-site competition curve. The distributio n between high-affinity (25%) and low-affinity (75%) sites suggested t hat both the alpha(1A)- and alpha(1B)-adrenoceptors were coupled to PI hydrolysis in proportion to their relative abundance. Equally, the st imulation of PI hydrolysis by epinephrine in the presence of a fixed c oncentration of 5-methyl urapidil was biphasic. In addition, chloroeth ylclonidine, an irreversible inhibitor of the alpha(1B)-adrenoceptor, inhibited the epinephrine stimulation of PI hydrolysis by 35%. we conc lude that the pharmacologically defined alpha(1A)- and alpha(1B)-adren oceptor subtypes are both coupled to PI hydrolysis in the ventricular myocyte.