Electrophysiologic Effects of ATP on Rat Ventricular Myocytes

Abstract

Since the original description of the effect of micromolar concentrations of extracellular ATP on the cytosolic calcium concentration ([ Ca2+],) of ventricular cells,' other groups have reported similar findings?' A common finding was that ATP stimulates calcium entry into cells from the extracellular solution primarily through dihydropyridine-sensitive calcium channels. It was not known, however, whether this was a direct or indirect action of ATP on these calcium channels.The whole-cell configuration of the patch-clamp technique was used to investigate the electrophysiologic effects of extracellular ATP on rat ventricular myocytes. The L-type calcium channel was isolated by depolarizing the cell from a holding potential (V,) of -45 or -40 mV to various testing potentials (VT). Outward potassium currents were inhibited by substituting cesium for potassium in the pipette filling solution and the extracellular solution. ATP (50-500 pM) had no effect on the current-voltage (I-V) relationship (N = 10) of the L-type current or on the peak current elicited by stepping to V, = 0 mV (N = 7). These results suggest that ATP does not directly effect the L-type calcium current.During the recording of the L-type calcium current, there was an increase in the inward holding current when ATP was applied to the cell. This phenomenon was similar to that described in smooth muscle? To record the ATP-activated current, cells were held at V, = -70 mV and the current was continuously monitored. FIGURE 1A is a representative experiment demonstrating the ability of 100 p M to produce an inward current. Soon after the application of ATP, a slowly activating, long-lasting inward current was elicited. The I-V relationship of the current shown in FIGURE 1B was generated by computer subtraction of the ramp (-100 to +30 mV) current before ATP application from the ramp current after ATP application. The current of each cell, at every 10-mV interval, was normalized to the current at -100 mV. The I-V relationship was linear and had a reversal potential of -1.9 2 6.5 mV (N = 12).Associated with the ATP-activated current was membrane depolarization. When cells were bathed in an extracellular solution devoid of voltage-sensitive channel blockers, ATP depolarized cells to the threshold for the firing of action potentials. (FIG. 2A). The first action potential was typical of a fast sodium-dependent action