Older individuals frequently manifest an increase in systolic blood pressure, arterial stiffening and reduced vasorelaxation in response to beta2 agonists like isoproterenol (ISO). Using a single cell model to investigate the mechanism of vasorelaxation induced by beta2 agonists, we observed alterations in vascular smooth muscle cell Ca2+metabolism during aging that lead to increased Ca2+ stores in the sarcoplasmic reticulum (SR) following application of ISO. These stores reduce the effective Ca2+ buffering capacity of SR and provide a source of contractile Ca2+ to increase vascular tone and peripheral resistance in the presence of circulating vasoconstrictors. In kidney, brain and vascular tissue, signal transduction by beta2 agonists involves activation of protei kinase A (PKA), although cyclic AMP-dependent activation of protein kinase G may also occur. We found that perfusion of rat tail artery cells with the PKA inhibitor, H89, failed to prevent the SR Ca2+-depleting effect of ISO, suggesting that cyclic AMP may mediate ISO-dependent Ca2+ redistribution by a PKG pathway. The events triggered by ISO in vascular smooth muscle involve a reduction in Ca2+ influx through L-type channels in the sarcolemmal (SL) membrane. This effect may involve stimulation of Ca2+-activated K+ channels, which hyperpolarize the membrane. Spontaneous release of Ca2+ from the SR via ryanodine-sensitive channels referred to as ~sparks~ may and induce membrane hyperpolarization by stimulating Ca2+-activated K+ channels. Using confocal microscopy, we have begun to examine whether ISO perfusion leads to an increased duration or frequency of sparking activity in rat tail artery smooth muscle cells. Activation of sparks by IS would provide a unitary mechanism to explain SR Ca2+ depletion and the reduction in Ca2+ influx that accompanies ISO application.
