EFFECTS OF HYDRALAZINE ON RESPONSES TO HISTAMINE AND CAFFEINE IN ILEAL LONGITUDINAL MUSCLE AND TAENIA-COLI OF THE GUINEA-PIG

1. The mechanism of tension inhibitory effects on histamine and caffei ne-induced responses by hydralazine were studied in the ileal longitud inal muscle and taenia coli of the guinea pig. Hydralazine, 10(-4) to 3 x 10(-3) M, inhibited both the tonic response and the increase in Ca uptake by the lanthanum (La) method evoked by histamine (10(-5) M) wi thout affect on Ca efflux, with smaller effects on the phasic response in ileal muscle. 2. Hydralazine suppressed caffeine- and IP3-induced contractions after loading with Ca2+ into the storage sites in saponin treated fibers. However, hydralazine suppressed the caffeine- and IP3 -induced contractions in the skinned fibers relatively less than both the transient response to caffeine and phasic response to histamine in intact muscles, respectively. 3. It is probable that hydralazine redu ces the tonic response to histamine mainly by inhibiting Ca2+ influx t hrough receptor operated Ca2+ channels in the ileum. In addition, hydr alazine also decreases caffeine- or IP3-induced Ca2+ release from stor age sites. Copyright (C) 1997 Elsevier Science Inc.