While a large body of evidence has demonstrated that cardiac responses to beta1- or mixed beta1-adrenergic receptor (betaAR) stimulation decrease with aging, the biophysical cellular mechanisms underlying the age-associated changes are not well understood. In addition, possible age effects on beta2RA stimulation is unclear. Our recent studies have shown that the distinct effects of betaAR subtype stimulation in cardiac myocytes are mediated by different signalling pathways. Specifically, beta2AR stimulated positive inotropic effect is dissociated from beta2AR-induced increase in cAMP. In addition, beta2AR is simultaneously coupled to Gs as well as a pertussis toxin (PTX) sensitive G protein. Therefore, beta1AR and beta2AR stimulation in heart cells may be differentially affected by aging. In the present studies, the ability of betaAR subtype stimulation to modulate cardiac Ca2+ metabolism and contractility was examined in single rat cardiac cells from hearts of a broad age range (2-4, 6-8, and 24 mo). Under control conditions, there was no systematic age difference in L-type Ca2+ current (Ica), cytoplasmic Ca2+ (Cai) transient or contraction. However, a generalized diminution in the response of all these parameters to a beta1AR agonist norepinephrine (NE) occurred with aging. In additional studies we found that the contractile response to beta2AR stimulation by zinterol (ZINT) is also markedly decreased with aging. Furthermore, PTX specifically potentiates the positive inotropic effect of beta2AR, but not beta1AR stimulation in both young and old groups (2 and 24 mo). However, the potentiating effect of PTX is not altered by aging, suggesting that the age-associated diminishment in betaAR subtype stimulated positive inotropic effect is not caused by a up-regulation of PTX- sensitive G protein(s) with aging. Taken together, the results of our studies show that the positive inotropic effects of both betaAR subytpes stimulation are decreased with aging. The age effects on betaAR subtype stimulation may be mediated by multiple mechanisms, including changes in receptor, Gs or betaAR-Gs-adenylyl cyclase coupling. For example, betaAR subtypes could be differentially desensitized in aged hearts by beta-adrenergic receptor kinase (betaARK) induced phosphorylation. Further studies are required to test these hypothesis.
