COMPARATIVE EFFECTS OF BRL-38227, NITRENDIPINE AND ISOPRENALINE ON CARBACHOL-STIMULATED AND HISTAMINE-STIMULATED PHOSPHOINOSITIDE METABOLISM IN AIRWAY SMOOTH-MUSCLE

1 The ability of BRL 38227 and nitrendipine to affect muscarine agonist and histamine-stimulated [H-3]-inositol phosphate accumulation in slices of bovine tracheal smooth muscle has been studied and compared with the established inhibitory effects of isoprenaline on this pathway. 2 Pre-addition of BRL 38227 (5-mu-M), nitrendipine (1-mu-M) or isoprenaline (10-mu-M) significantly inhibited the subsequent inositol phosphate response to histamine at all concentrations studied (10- 1000-mu-M). BRL 38227 and nitrendipine also significantly inhibited the [H-3]-inositol phosphate response to low (1-mu-M), but not high (100-mu-M) concentrations of carbachol. Isoprenaline had no effect at any concentration of carbachol studied. 3 Nitrendipine (IC50 = 95 nM) and BRL 38227 (IC50 = 322 nM) caused concentration-related inhibitions of the inositol phosphate response to histamine (100-mu-M). Similar maximal inhibitions were caused by each agent (55- 58%). Inhibitory effect of BRL 38227 was reduced in potency (IC50 = 5.5-mu-M), but not magnitude, in the presence of glibenclamide (0.5-mu-M). 4 Time-course studies comparing the effects of BRL 38227 addition 15 min before, and 10 min after histamine challenge showed that for pre-addition a distinct (< 2 min) lag occurred following histamine addition before the inhibitory effect of BRL 38227 was manifest. In contrast, when BRL 38227 was added 10 min after histamine, an inhibitory effect was immediately apparent. 5 Further evidence for an initial, 'protected' phase of inositol phosphate accumulation was provided by the finding that BRL 38227 pre-addition had no effect on the early (0-300 s) time-course of inositol 1,4,5-trisphosphate mass accumulation. 6 The inhibitory effect of BRL 38227, but not that of nitrendipine or isoprenaline, on histamine-stimulated [H-3]-inositol phosphate accumulation was completely prevented in the presence of an elevated extracellular K+ (65 mM) concentration. 7 The results demonstrate that membrane hyperpolarization, and/or blockade of voltage-operated Ca2+-channels can regulate agonist-stimulated phosphoinositide metabolism in airway smooth muscle. The possible contribution of this regulatory mechanism to the relaxant properties of these agents is discussed.