VASCULAR SMOOTH-MUSCLE CONTRACTILE PROPERTIES IN THE PIGEON

Rings excised from the upper portion (aortic arch to A. Celiaca) and lower portion (A. celiaca to A. renalis cranialis) of the pigeon aorta showed distinctly different contractile properties, Both tissues exhibited significant temperature sensitivity over the range of 45-22 degrees C with optimal temperature at 40-42 degrees C. The optimal preload was approximately 3 g and 8 g for the lower and upper segments of the aorta, respectively. The two segments differed significantly in their sensitivity to potassium depolarization. Disruption of the endothelium did not alter the sensitivity to potassium depolarization, but reduced the maximal response. The response of both segments to norepinephrine was quite variable, and showed evidence of decreased responsiveness with repeated contraction or prolonged exposure to the compound. Both segments showed extracellular calcium dependency. Significant tension, development in zero calcium buffer, was observed for both segments, but was particularly pronounced in the upper aorta. Cytochalasin D, an inhibitor of actin polymerization, blocked slow tension increase in both segments, but was particularly effective in rings from the lower segment of aorta. The results indicate that the regulation of contractile function in pigeon aorta differs in important respects from that of mammalian tissue.