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Cylindrical segments of carotid arteries from rat, rabbit, and dog were studied in vitro in order to compare active and passive mechanical properties, with gross morphology and composition. Pressure-diameter relations were determined under active (norepinephrine) and passive (2 mM EGTA) conditions, and were used to compute values of tangential wall stress, incremental elastic moduli, characteristic impedance, and active smooth muscle responses. The water and connective tissue contents of these segments were also determined. Significant differences were found in the passive mechanical properties of these arteries, with those of the rat being most compliant and those of the dog being the stiffest. These differences were consistent with the connective tissue content of these arteries. The ratio of collagen to elastin was smallest in the rat and largest in the canine carotids. Differences were also demonstrated in the responses to smooth muscle activation. The maximum value of active stress response was essentially the same in the canine and rabbit arteries, both of which were larger than that of the rat carotids. On the other hand, the rabbit vessels produced a larger maximum diameter response than either of the other arteries. Values of incremental elastic modulus were largest at specific values of transmural pressure for the rabbit vessels. This latter fact may make the contractile system more effective in reducing wall diameter in the rabbit arteries in spite of the fact that they did not have the capacity to generate a larger active stress response.
R. H. Cox (Wed,) studied this question.