Constriction of Ear Arteries from Normotensive and Renal Hypertensive Rabbits against Different Transmural Pressures

Abstract

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.

The interaction between noradrenaline activation and distension activation of the rabbit ear artery.

“…There may well be more than 500000 muscle cells in a 10 mm long segment of the central ear artery. The number of layers of muscle cells making up the media is about eleven (Speden & Ryan, 1982) and the mid-wall circumference of the relaxed, distended arteries averaged 4020 jm in the current experiments. Muscle cells in the wall of relaxed, distended ear arteries had a mean length of 107,sm which gives an estimate, assuming end-to-end packing of cells, of 413 cells per transverse cross-section of artery.…”

“…Release of endogenous substances such as endothelium-derived relaxing factors (Griffith, (Speden & Ryan, 1982) and the mid-wall circumference of the relaxed, distended arteries averaged 4020 jm in the current experiments. Muscle cells in the wall of relaxed, distended ear arteries had a mean length of 107,sm which gives an estimate, assuming end-to-end packing of cells, of 413 ).…”

“…The main evidence for myogenic control ofthe diameter ofactive ear arteries is their insensitivity to physiological changes in transmural pressure (Speden, 1973(Speden, , 1975Speden & Ryan, 1982) and their ability to react to distension by a pressure jump with a counteracting constriction . Other isolated, pressurized arteries (Hinke & Wilson, 1962;Cox, 1978a, b) and arterioles (Duling, Gore, Dacey & Damon, 1981;Dacey & Duling, 1982) also show a high resistance to distension when active.…”

Active reactions of the rabbit ear artery to distension.

“…An estimate of the contractile performance of the muscle during these active reactions to pressure changes can be obtained by subtracting the passive circumferential wall stress (force/wall cross-sectional area) from the total circumferential wall stress. The difference gives an indication of the force imposed upon the muscle (Speden, 1975;Speden & Ryan, 1982). These subtractions have been done for the active reactions shown in Figs.…”

“…The blood pressure in the ear artery of conscious rabbits was found to be 94/66 mmHg (mean systolic/mean diastolic) in earlier experiments with the upper limit for effective constriction of the isolated artery being 120 mmHg (Speden & Ryan, 1982). The base-line transmural pressure was therefore set at 60 mmHg and Fig.…”