Isotonic contractions were measured in helically cut strips of canine main pulmonary artery (MPA). Applied tension/unit area was of the order of magnitude of average tangential stress calculated for MPA wall in vivo. Angiotensin was found to be a highly effective vasoconstrictor in this preparation. Epinephrine, norepinephrine, phenylephrine, metaraminol, methoxamine, mephentermine and 5-hydroxytryptamine also produced significant vasoconstriction, while oxytocin and vasopressin were ineffective. Aeration with 100% CO 2 resulted in slight vasodilation, followed by progressive vasoconstriction after pH fell below 6.0. Maximum shortening (S max ) was elicited by l-epinephrine in preparations under tensile stress of 6.4 x 10 4 to 16.5 x 10 4 dynes/cm 2 . S max (average 20%) was directly related ( r = 0.78, P < 0.01) to the amount of elongation which took place during equilibration of strips under tension. The results suggest the feasibility of active regulation of MPA geometry by smooth muscle.
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Somlyo et al. (1964) studied this question.