Modulation of electrical and contractile activity by catecholamines in aging ventricular myocytes

Spring R. Farrell, Gregory R. Ferrier and Susan E. Howlett. Department of Pharmacology, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada B3H 1X5.

The sensitivity of the heart to adrenaline and other catecholamines declines with age (Lakatta et al., 1975). However, it is not clear whether reduced sensitivity to catecholamines alters the pro-arrhythmic effects of β adrenergic receptor (βAR) stimulation in the aging heart. The objectives of this study were: 1) to determine and compare the contractile responses of young adult and aged myocytes to βAR stimulation by the non-selective βAR agonist, isoproterenol (ISO); 2) to determine whether induction of aftercontractions, a sign of calcium overload, by ISO, is altered in aged myocytes; and 3) to determine whether induction of the arrhythmogenic transient inward current (ITI) by ISO is altered in aged myocytes.

Young adult (3 mos; 293 ± 6 g) and aged (24 mos; 421 ± 8 g) Fischer 344 male rats were anaesthetised with an overdose of sodium pentobarbital (25 mg/kg). Hearts were removed and ventricular myocytes were isolated enzymatically as described previously (Ferrier et al., 2003). Myocytes were superfused with a HEPES-buffered solution (pH 7.4) maintained at 37C o. In all experiments, unloaded cell shortening was measured with a video edge detector. Contractions were measured in field-stimulated myocytes, while transmembrane currents were measured with voltage clamp techniques. Differences were tested with repeated measure (RM) ANOVA, t-tests or χ2 tests. Results are expressed as means ± SEM.

The inotropic response to 10-7 M ISO was reduced in aged myocytes (13.86 ± 1.39 µm vs. 8.69 ± 0.73 µm for young adult and aged cells, respectively; p<0.05, 2 way RM ANOVA; n=11-12 cells/group). In addition, the concentration-response curves for contraction were significantly shifted to the right in the aged group (EC50 = 5.12 ± 2.48 nM vs. 39.42 ± 8.82 nM in young adult and aged myocytes, respectively; p<0.05, t-test; n=5-9 cells/group). The percentage of cells exhibiting aftercontractions induced by 10-7 M ISO was 91.67% in young adult and 9.52% in aged myocytes (p<0.05, χ2 ; n=12-21 cells/group). In voltage clamp experiments, cells were held at -80 mV, depolarised to +20 mV and then repolarised to -70 mV to observe ITI. Exposure to 10-7 M ISO induced ITI in young adult cells more frequently than in aged myocytes (10-7 M ISO: 54.14% vs. 7.14% in young adult and aged myocytes, respectively; p<0.05, χ2 ; n=7-14 cells/group).

These results demonstrate that the reduction in sensitivity to βAR stimulation was accompanied by a reduction in the pro-arrhythmic effects of adrenergic stimulation in ventricular myocytes from aging heart. Thus, reduced sensitivity to catecholamines may serve to protect the aging heart from cardiac arrhythmias triggered by ITI.

Ferrier GR et al. (2003) Am J Physiol 285, H1495-H1505.
Lakatta EG et al. (1975) Circ Res 36, 262-269.