Beta-receptor activity in aorta; variations with age and species.

Helically cut thoracic aortic strips (from rats, rabbits, and guinea pigs) in a state of low to moderate tone and under the influence of alpha-receptor blockade relaxed in response to isoproterenol. Propranolol blocked these relaxations. Helically cut thoracic aortic strips from cats and abdominal aortic strips from rats, rabbits, and cats were not relaxed by isoproterenol. In addition, aortic beta-receptor activity of rats and rabbits decreased with increasing age. The ability of thoracic aortas to be relaxed by isoproterenol was lost when rats were 90 days old and when rabbits were 2 years old. Significant loss of beta-receptor activity was not evident in rat trachea and stomach. Sodium nitrite completely relaxed thoracic and abdominal aortas from all species including thoracic aortas from old rats and rabbits. We conclude: (1) there are species differences in the response of thoracic aortic strips to beta-receptor stimulation; (2) a gradient in beta-receptor activity exists in the aorta with greater activity in the thoracic aorta and a relatively small amount of activity in the abdominal aorta; and (3) aortic beta-receptor activity decreases with increasing age. ADDITIONAL KEY WORDS isoproterenol trachea stomach thoracic aorta abdominal aorta cat guinea pig rat rabbit • Marked species and age differences in the response to drugs are well known (1-4). Although variations in rates of drug metabolism may account for many of these differences , little is known about species and age differences at the receptor level. The presence or absence of specific drug receptors in certain organ systems might be the major or a contributory factor underlying species and age differences. This paper compares beta-receptor activity in aortic strips from various laboratory animals. The results show that beta-receptor activity is present in thoracic aortas of guinea pigs, rabbits, and rats but is absent in those of cats. Furthermore, in rabbits and rats the activity of beta receptors in the thoracic aorta is greater than in the corresponding abdominal region. In addition, the results indicate that during the aging process, the thoracic aorta loses its ability to relax in response to beta-receptor stimulation but does not lose its ability to relax in response to NaNO^, though it becomes less sensitive to this drug. Methods and Materials Animals of either sex were used. Guinea pigs (NIH strain) weighed 400 to 800 g, rabbits (New Zealand) 1.5 to 3.3 kg; rats (Sprague-Dawley) 150 to 550 g, and cats 1.2 to 4 kg. …