Influence of initial length on length-tension relationship of vascular smooth muscle.

relutionshi~ of vascular smooth muscle. Am. J Physiol. 2 2 5 (3) : 664-670. 1973.-Segments of dog carotid artery were excised, cannulated, and restored to in situ length. They were immersed in a KrebsRinger bath and inflated with 100% 02 under nonoscillating pressures. Diameter was continuously monitored with a linear displacement transducer. The vessel segments were relaxed, then treated with norepinephrine to excite the muscle. Some vessels were excited isometrically and others were excited isobarically. Following full contraction, the pressure was decreased in 25-mm Hg steps to 0 mm Hg to permit graded vessel contraction. With some vessels contracted to 0 mm Hg, the pressure also was reelevated in steps. Then the muscle was inactivated with potassium cyanide, and the mechanical properties of the connective tissues were measured. Finally, the vessel wall volumes were computed from roentgenographic measurements of the vessel walls. Strainactive stress curves were computed from these data. Maximum active smooth muscle isometric stress was I. 1 X I O6 dynes/cm2 for the whole vessel wall or 2.4-3.5 x IO6 dynes/cm2 when corrected for the percent of the wall occupied by smooth muscle. Activation of the muscle at pressures of 150 mm Hg or less and circumferential strains of .70 or less gave reproducible strain-active stress curves. Excitation at higher pressures and larger strains elicited depressed length-tension curves. Distention from 0 mm Hg of vessels exhibiting decreased active stress revealed total recovery of active stress. At large initial strains, isometric contraction caused greater attenuation of active stress than did isobaric contraction. A final group of vessels was excited isometrically at 100 and 230 mm Hg. Distention of these vessels following isometric contraction indicated that the attenuation of the strain-active stress curve following isometric contraction at high pressure and large strains was due to a decrease in the strength or number of contractile units and not to slippage within the muscle or vessel. wall.