The effects of isoproterenol on abnormal electrical and contractile activity and diastolic calcium are attenuated in myocytes from aged Fischer 344 rats

We investigated whether the age-related decrease in sensitivity of the heart to catecholamines was accompanied by changes in Ca²⁺ homeostasis and abnormal electrical and contractile activity caused by β-adrenergic receptor (β-AR) stimulation. Ventricular myocytes were isolated from young adult (3 months) and aged (24 months) male Fischer 344 rats. Unloaded cell shortening was measured in field-stimulated myocytes (2 Hz, 37 °C); membrane currents and action potentials were measured with microelectrodes. Contractile responses to the non-selective β-AR agonist, isoproterenol were significantly decreased in aged myocytes compared to younger myocytes and aged myocytes were less sensitive to isoproterenol. In contrast, Ca²⁺ transients measured simultaneously with contractions were similar between groups. Isoproterenol increased sarcoplasmic reticulum Ca²⁺ stores in both groups, but the increase was larger in aged cells. However, signs of Ca²⁺ overload induced by isoproterenol were reduced with age. Diastolic Ca²⁺ accumulation, contracture and the incidences of transient inward current, oscillatory afterpotentials (OAPs), aftertransients and aftercontractions induced by isoproterenol also were reduced with age. These results demonstrate that aged myocytes exhibit fewer signs of Ca²⁺ overload in response to isoproterenol than young adult myocytes. These age-related changes in intracellular Ca²⁺ may protect the aging heart against induction of arrhythmias initiated by OAPs.¹.