ACTIVATION OF BETA-ADRENERGIC RECEPTORS INHIBITS CA2-MEDIATED GENERATION OF INOSITOL PHOSPHATES IN THE GUINEA-PIG MYOMETRIUM, A CYCLIC AMP-INDEPENDENT EVENT( ENTRY)

In the guinea pig myometrium, carbachol, oxytocin, and fluoroaluminate s stimulated the breakdown of phosphatidylinositol 4,5-bisphosphate, w hich was insensitive to pertussis toxin [Biochem. J. 255:705-713 (1988 )]. We now demonstrate that an increased accumulation of inositol phos phates, with an early production of inositol 1,4,5-trisphosphate [Ins( 1,4,5)P3], could also be obtained with K+ (30 mM) and the Ca2+ inophor e ionomycin. Removal of extracellular Ca2+ or addition of the Ca2+ cha nnel antagonists nifedipine and verapamil almost totally abolished sti mulations elicited by high K+ and partially attenuated receptor- and f luoroaluminate-mediated increases in inositol phosphates. Isoprotereno l similarly attenuated the accumulation of inositol phosphates elicite d by carbachol, oxytocin, and fluoroaluminates (maximal inhibition, 35 %; EC50, 0.5 nM), with no change in the rate of Ins(1,4,5)P3, inositol bisphosphate, and inositol monophosphate generation. The beta-adrener gic receptor-induced inhibition was prevented by pertusis toxin and co uld not be reproduced by forskolin, indicating that cAMP was not invol ved. Experimental findings were, rather, consistent with a predominant role for Ca2+. Thus, inhibition due to isoproterenol was lost in a Ca 2+-depleted medium and was not additive with that caused by nifedipine . Accumulation of inositol phosphates triggered by high K+ was insensi tive to the beta-adrenergic receptor inhibition. The inhibitory effect of isoproterenol, similar to that of nifedipine, was counteracted by ionomycin and also by the Ca2+ channel agonist Bay K 8644. These data indicate that in the myometrium 1) phospholipase C can be activated th rough a voltage-gated Ca2+ entry-dependent process that contributes at least partially to the stimulations triggered by receptor- and/or gua nine nucleotide-binding protein-mediated activation and 2) beta-adrene rgic receptor activation is linked via a cAMP-independent, pertussis t oxin-sensitive pathway to an inhibition of voltage-gated Ca2+ channels , resulting in an attenuation of the Ca2+-associated generation of ino sitol phosphates.