The rate and force of the heart's condition are regulated by the autonomic nervous system and by circulating hormones. A primary mechanism of regulation is modulation of the number and behavior of ionic channels in cardiac cell membranes. The goal of the proposed research is to use newly available methods of ionic channel biophysics to understand how the operation of cardiac channels is changed by hormonal stimulation. With a primary emphasis on regulation of the channels carrying slow inward calcium current, the experimental approach will combine patch clamp recordings of currents through single calcium channels with gigaseal electrode recordings of currents from whole cells, using fluctuation analysis techniques to estimate changes in the number of functional calcium channels in a cell. The ability to dialyze the inside of a single cell will permit investigation of the possible role of internal calcium as a second messenger. These approaches will help provide answers to basic questions about calcium channel regulation. Do Beta-adrenergic agonists increase calcium current by recruiting an entirely new population of channels or by modifying the operation of a single class of channels? How are calcium channel kinetics, voltage-dependence, and sensitivity to channel blockers changed by adrenergic stimulation? Does acetylcholine regulate calcium channels by a direct or a second messenger action? Are changes in internal calcium important for acetylcholine's action? How does angiotensin's regulation of the calcium channel differ from that of adrenaline? Does adenosine decrease calcium current by a common mechanism with acetylcholine? Two specific questions about potassium channel regulation will also be studied. In adrenergic increases of voltage-activated potassium channels, are new channels recruited or old channels modified? Does adenosine induce resting potassium current by a second messenger system? Neurotransmitter and hormonal regulation of calcium and potassium channels is important not only for the normal physiological control of the heart, but also in the genesis of some arrhythmias. The proposed experiments may also shed light on related physiological control systems in exocrine and endocrine cells, smooth muscle, and neurons.