Modulation of bradykinin-induced gastric-cardiovascular reflexes by histamine

Both histamine and bradykinin induce gastric-cardiovascular reflexes and are released during several pathophysiological conditions. This study examined the possibility that histamine modulates the magnitude of the reflex response to stimulation by bradykinin. Thus in chloralose anesthetized cats, the cardiovascular response to stimulation of the gastric serosa with 1 microgram/ml bradykinin was monitored before and after topical application of 100 micrograms/ml histamine (n = 6) or 1 mg/ml diphenhydramine (H1-receptor antagonist) and histamine (n = 5). After application of histamine, bradykinin-induced increases in mean arterial pressure and left ventricular pressure were attenuated by 23 and 27%, respectively. Conversely, when the H1-receptors on the serosal surface of the stomach were blocked (n = 5) before application of histamine, the pressor response to bradykinin was augmented by 26%. To determine the afferents that might contribute to the attenuating effect of histamine, we recorded single unit activity in 14 A delta and 21 C visceral afferent fibers in response to bradykinin stimulation before and after histamine stimulation. We observed that the impulse activity of 10 of the A delta and 14 of the C fibers to bradykinin stimulation was reduced after treatment with histamine. These results suggest that histamine induces an inhibitory effect on the nerve endings of visceral A delta and C fibers to the action of bradykinin through an H1-receptor mechanism. This inhibitory effect attenuates the magnitude of the consequent cardiovascular reflex response.