The Role of Gi-proteins andβ-Adrenoceptors in the Age-related Decline of Contraction in Guinea-pig Ventricular Myocytes

A decline in contractility in myocytes from ageing guinea-pig hearts was demonstrated, which is more pronounced for maximumβ-adrenoceptor-stimulated activity than contraction in high Ca2+. In this study the role of the inhibitory G-proteins (Gi) in this process was investigated. Comparisons were made between young (Y, <400 g, <4 weeks), adult (A, >600 g, >8 weeks) and senescent guinea pigs (S, 53–65 weeks, 1136±30 g). Giαactivity, detected by pertussis toxin-catalysed ADP ribosylation, was significantly increased in senescent compared to young animals, but immunodetectable levels of Giαwere unchanged.β-adrenoceptor number was decreased by 27% in senescent compared with young animals (P<0.002). Pertussis toxin treatment increased the maximum response to isoproterenol in contacting myocytes so that there was no longer any significant decline with age. Maximum contraction amplitudes (sarcomere length change,μm) with isoproterenol before pertussis toxin were 0.144±0.011 (Y,n=22 animals), 0.104±0.009 (A, 18) and 0.098±0.009 (S, 14),P<0.01 by analysis of variance (ANOVA). Following toxin treatment amplitudes were 0.140±0.012 (Y, 12), 0.117±0.010 (A, 10) and 0.117±0.018 (S, 8),P=n.s. Pertussis toxin treatment also reversed the effects of ageing on contraction and relaxation velocity in isoproterenol. In contrast, the effect of age on contraction amplitude or velocity in maximum Ca2+was more pronounced after toxin treatment. The EC50value for isoproterenol increased with age: pertussis treatment decreased the EC50in each group, but the effect was especially pronounced for senescent animals. There was no significant difference in the concentration–response curves for the negative inotropic effect of adenosine (in the presence of isoprotenerol) between the three age groups before toxin treatment. All effects of adenosine were abolished after pertussis exposure. We conclude that increased Giαactivity is likely to contribute to the decreased response to isoproterenol, but not to high Ca2+, in myocytes from ageing guinea-pigs.