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Isolated segments of rabbit ear arteries were made to constrict against normotensive and hypertensive transmural pressures by perfusion with submaximal concentrations of norepinephrine (NE). Changes in load (force/unit length of artery) and stress (force/wall cross-sectional area) during constriction against a constant pressure have been evaluated. Weak concentrations of NE constricted the arteries equally well against transmural pressures of 80 and 120 mm Hg and, in doing so, utilized much of the contractile capacity of the muscle. A stretch-mediated, co-operative interaction between muscle cells has been put forward to explain these observations. Ear arteries from renal hypertensive rabbits differed from those of normotensive rabbits in having a higher NE threshold concentration and in constricting better against 140 mm Hg. They did so because of the mechanical advantage provided by a smaller internal radius and a thicker wall which reduced the load and stress placed on the muscle by the pressure. No muscle hyperplasia or hypertrophy was detected.

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The size of motor units in rat laryngeal muscles was determined by correlating the number of neurons labeled by i.m. injections of horseradish peroxidase with the number of motor end plates stained for acetylcholinesterase. The cricothyroid has a motor unit size of 8 muscle fibers per motor neuron and the posterior cricoarytenoid 4-5 muscle fibers per motor neuron.