The present study was undertaken to determine whether the phosphoinositide hydrolysis is responsible for the positive inotropic effect of histamine in guinea-pig left atria. Histamine induced hydrolysis of phosphoinositides and a positive inotropic effect in a concentration-dependent manner. These effects were antagonized by chlorpheniramine (0.1 mumol/l) but not by cimetidine (10 mumol/l). At a concentration of 1 mumol/l histamine produced a dual-component positive inotropic response composed of an initial increasing phase and a second and late developing, greater positive inotropic phase. Histamine (10 mumol/l) caused a gradual increase in the formation of [3H]inositol trisphosphate (IP3) and a significant increase in the [3H]IP3 level was detected 10 min after the stimulation. Thus, the increase in IP3 did not precede the increase in force of contraction. The phospholipase C inhibitors 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (100 mumol/l) and neomycin (100 mumol/l) significantly reduced the histamine-induced [3H]inositol monophosphate accumulation. However, pretreatment with the phospholipase C inhibitors did not affect the positive inotropic effect of histamine, either in its extent or in its pattern. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) (100 nmol/l) and phorbol-12,13-dibutyrate (PDBu) (100 nmol/l) also significantly inhibited the phosphoinositide hydrolysis induced by histamine. The inhibitory effect of the phorbol esters on the phosphoinositide response was completely abolished in the presence of 10 mumol/l 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), a protein kinase C inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)