We desired 02 concentrations. Po2 in the chamber was monitored with an O2-sensing electrode (9). Macroscopic calcium currents were studied using the whole-cell configuration of the patch-clamp technique (3,13) and recorded in isolation after blockade of the voltage-dependent Na+ and K+ channels. Cytosolic [Ca2W] was estimated in unclamped cells loaded with fura-2 by incubation for 10 min at 37°C with saline containing 1 AM fura-2 acetoxymethyl ester. Experiments were performed on an inverted microscope with standard optical components and equipped for epifluorescence and photometry (14). For the two excitation wavelengths, we used the filters shortwave-pass SWP 357 (excitation at =360 nm) and band-pass BP 380 (excitation at 380 nm; bandwidth, 10 nm). Fluorescence from the cells was measured by a dual wavelength photometer. The two output voltage signals from the photometer were digitized and displayed on-line on the screen of a computer in parallel with the estimated [Ca2+] concentration (15). Calibration of the fluorescence signals in terms of [Ca2+] was performed in vitro as described (16). Secretion was monitored in amperometric mode with a glass-sealed 8-,um diameter carbon electrode fabricated as described (17)(18)(19)(20). In most experiments, we used an amplifier built in our laboratory that has in the headstage a Burr-Brown OPA 111 wired as a current-to-voltage converter with a feedback resistor of 500 MQ1. The high-resolution recordings (see below) were obtained with a standard List EPC-7 patch-clamp amplifier. The single events of high resolution, similar to those shown in Fig. 2C, were recorded with a carbon electrode covered with polyethylene, which, as shown before (19), decreases noise and permits the acquisition of signals at a broader bandwidth. We held the carbon fiber at a constant voltage of +950 mV, a potential more positive than the oxidation potential of dopamine, thus assuring the oxidation of dopamine released by the cells (see below). Cyclic voltammograms were obtained by applying voltage ramps from -600 to + 1000 mV (at a rate of 170 V/s) to the carbon-fiber electrode. The signals are characterized by typical reduction (at approximately -400 mV) and oxi-*To whom reprint requests should be addressed. 10208The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

The effect of dopamine on the voltage-dependent ionic channels of enzymatically dispersed glomus cells from rabbit carotid bodies was studied. Whole-cell currents were recorded on isolation with patch electrodes and dopamine applied to the bath solution. Dopamine at nanomolar concentrations produced a reversible attenuation of the calcium current whereas sodium and potassium currents remained unaltered. Dopamine inhibition of Ca2+ current was observed in all cells tested (n=48) and at a saturating concentration (1 microM) the average reduction was of 40 +/- 6.5% (n=8). The effect of dopamine was probably caused by a decrease in the number of channels activatable on depolarization since it did not modify the voltage-dependent parameters of the current. These results indicate that dopamine, which is the major transmitter secreted by glomus cells, regulates further transmitter release by feedback inhibition of Ca2+ channels.

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