Responsiveness of longitudinal and circular muscle layers of the portal vein.

This study was performed to compare mechanical activity produced by acetylcholine and histamine in longitudinal and circular muscle layers of the portal vein. Peak tension was measured in longitudinal and circular muscle strips from portal veins of rabbits as they were superfused with acetylcholine and histamine (10(-6) to 10(-4) M). Responses to acetylcholine were three to four times greater in longitudinal muscle than in circular muscle; for example, acetylcholine (10(-5) M) produced contractions of 1.6 +/- 0.2 X 10(3) (mean +/- SE) dynes in longitudinal muscle and 0.4 +/- 0.09 X 10(3) dynes in circular muscle. In the same strips, histamine produced similar responses in the two layers: histamine (10(-5) M) produced contractions of 0.4 +/- 0.1 X 10(3) dynes in longitudinal and 0.4 +/- 0.1 X 10(3) dynes in circular muscle. Histological studies of portal vein muscle strips showed that the longitudinal muscle layer is three to four times thicker than the circular muscle layer. Thus, in response to acetylcholine, tension developed in each muscle layer is proportional to the thickness of that layer. In response to histamine, however, longitudinal muscle develops less tension than is predicted by the thickness of the muscle layer. We conclude that longitudinal and circular muscle layers of the rabbit portal vein differ in their responsiveness to histamine.