Two types of Ca2+ currents are found in bovine chromaffin cells: facilitation is due to the recruitment of one type.

Abstract

1. Whole‐cell Ca2+ currents in cultured bovine chromaffin cells were studied using patch‐clamp electrophysiology. With Ba2+ or Ca2+ as the current carriers, two separate components of whole‐cell current could be distinguished by biophysical and pharmacological criteria. These components of Ca2+ current were different from T‐ or N‐type Ca2+ channels previously described, as they were not inactivated at a holding potential of ‐60 mV. 2. Depolarization of the cells past ‐20 mV in 10 mM‐Ba2+ activated a single component of Ca2+ current, called the 'standard’ current. This current showed no detectable voltage‐dependent inactivation, but did show marked current‐dependent inactivation as steady‐state inactivation (H‐infinity) plots obtained in the presence of Ba2+ were quite different from those obtained from Ca2+. 3. In most chromaffin cells large pre‐depolarizations or repetitive depolarizations in the physiological range activated a second component of Ca2+ current called ‘facilitation’. Facilitation was observed with either Ca2+ or Ba2+ as the charge carrier. Recruiting facilitation increased whole‐cell currents by an average of 60%. 4. Pre‐pulses to +120 mV lasting 200 ms completely activated facilitation. Pre‐pulses longer than 800 ms started to inactivate facilitation, while pre‐pulses longer than 2500 ms completely inactivated this component of Ca2+ current. Because only outward currents were recorded at +120 mV, it is likely that facilitation inactivated in a voltage‐dependent manner. 5. When the extracellular Ba2+ concentration was increased in the range from 2 to 90 mM activation of both facilitation and standard Ca2+ currents shifted in the depolarizing direction. In 2 mM‐Ba2+ facilitation activated at potentials 10 mV more negative than the standard component, while in 90 mM‐Ba2+, facilitation activated at a potential about 10 mV more depolarized than the standard component. Thus, the voltage sensor for the facilitation Ca2+ current appeared to sense more surface charge than did the standard Ca2+ current. 6. Tail currents measured at ‐20 and ‐30 mV in the absence of facilitation (without pre‐pulses) showed one time constant for current deactivation. Tail currents measured with both facilitation and standard currents activated showed a significantly slower deactivation rate than that seen with the standard current alone. 7. The dihydropyridine antagonist nisoldipine (1 microM) completely suppressed the facilitation Ca2+ current even when cells were held at negative holding potentials (‐80 mV). In contrast, the standard current was unaffected by 1 microM‐nisoldipine, even at depolarized holding potentials (‐20 mV).(ABSTRACT TRUNCATED AT 400 WORDS)