The age-related decrease in catecholamine sensitivity is mediated by ß₁-adrenergic receptors linked to a decrease in adenylate cyclase activity in ventricular myocytes from male Fischer 344 rats

This study determined whether reduced sensitivity to catecholamines in aged myocytes resulted from deficits in the ß-adrenergic receptor (ß-AR) signaling pathway. Contractions and intracellular Ca²⁺ were measured simultaneously in field-stimulated (2 Hz, 37 °C, fura-2) ventricular myocytes isolated from young adult (∼3 months) and aged (∼24 months) male Fischer 344 rats. Higher concentrations of a ß₁-AR agonist were required to increase contraction amplitudes in aged compared to younger cells; however, Ca²⁺ transients were similar in both groups. There was no age-related difference in contraction or Ca²⁺ transient amplitudes in response to a ß₂-AR agonist. The direct adenylate cyclase agonist forskolin caused smaller increases in contraction and Ca²⁺ transient amplitudes in aged compared to younger cells. Phosphodiesterase inhibitors did not reverse the age-related deficit in positive inotropy caused by ß-AR stimulation. Direct measurement of cAMP showed significantly less cAMP formation in response to either ß-AR or adenylate cyclase stimulation in aged compared to younger cells. However, responses to dibutyryl cAMP were similar in young adult and aged myocytes, suggesting that events downstream of cAMP formation are not affected by age. The age-related decrease in catecholamine sensitivity is mediated by ß₁-ARs, resulting in a defect in cAMP production.