Selectivity of histamine for the proximal muscle loop of the cat pylorus

Most drugs stimulate tonic and phasic muscular activity of the gastroduodenal junction to similar degrees except for histamine which enhances purely phasic contractions. The present study examined whether this selective histamine effect is due to a preferential action of histamine on the proximal muscle loop of the pylorus (PPL) which, unlike the distal loop (DPL), generates mostly phasic contractions. Strips were cut so as to represent the mechanical activity of muscle from the antrum, the PPL and the DPL of the cat stomach. All strips responded to acetylcholine with a combination of tonic and phasic activity. Histamine increased the frequency and the amplitude of phasic contractions in most strips from the antrum and the PPL; it increased the tonic contraction of a few DPL strips. In all types of strips, maximal histamine effects were only a fraction of the effects occurring with acetylcholine. Exposure to histamine produced no mechanical response in one fifth of the antral strips and in more than one third of the strips from both the PPL and the DPL. Histamine tachyphylaxis was marked, and sometimes irreversible. Histamine also led to phasic interruptions of flow across the isolated gastroduodenal junction without altering baseline flow. The present study demonstrates that histamine acts preferentially on those parts of the gastroduodenal junction that generate phasic rather than tonic muscular activity.