Age-dependent alterations in beta-adrenergic responsiveness of rat detrusor smooth muscle

The relaxant effects of norepinephrine (NE, 10(-7) to 10(-4) M.) and isoproterenol (ISO, 10(-9) to 10(-4) M.) on maximal KCl-induced tonic contractions and the relaxant effects of ISO on contractions induced by electrical field stimulation (EFS) were measured in detrusor muscle strips obtained from 22-25 day, 90-95 day and 22-month-old male Fischer 344 rats. The maximum relaxant response to NE and ISO on KCl-induced tonic contractions decreased significantly with increasing age. The ED50 values for ISO, but not NE, increased with age. The maximum relaxant response to ISO on EFS-induced contractions also was reduced significantly in the old bladders. The relaxation effects of forskolin (10(-6) to 3 x 10(-5) M.), dibutyryl cyclic AMP (DBcAMP, 10(-4) to 3 x 10(-3) M.) and cholera toxin (10 micrograms/ml.) were examined on maximal KCl-induced contractions of the muscle strips obtained from the three age groups. The relaxant responses to forskolin decreased significantly with increasing age, whereas DBcAMP relaxed the muscle strips from the three age groups equally. Cholera toxin (10 micrograms) attenuated KCl-induced phasic contractions, and this effect was impaired in the aged rat detrusor. The density of beta-adrenergic receptors, as determined by radioligand binding with [125I]iodopindolol ([125I]-PIN) decreased with increasing age. These data demonstrate an age-related decrease in the responsiveness of the bladder detrusor to beta-adrenergic stimulation that may be related to the decreased density of beta-adrenergic receptors and decreased cyclic AMP (cAMP) production.