Title: Dihydropyridine‐sensitive low‐threshold calcium channels in isolated rat hypothalamic neurones.
Abstract: 1. Low‐voltage‐activated Ca2+ channels which produce a transient inward current were studied in neurones freshly isolated from the ventromedial hypothalamic region of the rat. Membrane currents were recorded using a suction‐pipette technique which allows for internal perfusion under a single‐electrode voltage clamp. A concentration‐jump technique was also used for rapid drug application. 2. In most cells superfused with 10 mM‐Ca2+, a transient inward Ca2+ current was evoked by a step depolarization to potentials more positive than ‐65 mV from a holding potential of ‐100 mV. Such a low‐threshold Ca2+ current could easily be separated from a high‐threshold, steady type of Ca2+ current by selecting the holding and test potential levels, as well as by resistance to the wash‐out during cell dialysis. 3. Activation and inactivation processes of the low‐threshold Ca2+ current were highly potential dependent at 20‐22 degrees C. For a test potential change from ‐60 to +20 mV, the time to peak of the current decreased from 45 to 9 ms, and the time constant of the current decay decreased from 90 to 40 ms. The steady‐state inactivation occurred at very negative potentials, reaching a 50% level at ‐93 mV. Recovery from inactivation showed a time constant between 2.63 and 0.94 s for a potential change from ‐80 to ‐120 mV. 4. The amplitude of the low‐threshold Ca2+ current depended on the external Ca2+ concentration [( Ca2+]o), approaching saturation at 100 mM [Ca2+]o. Ba2+ substituted for Ca2+ reduced the current amplitude by 30‐50% while Sr2+ produced no definite changes in the current amplitude. 5. The low‐threshold Ca2+ current was blocked by various di‐ or trivalent cations in the sequence of La3+ greater than Zn2+ greater than Cd2+ greater than Ni2+ greater than Co2+. The corresponding apparent dissociation constants (KD) were 7 x 10(‐7), 1 x 10(‐4), 3 x 10(‐4), 6 x 10(‐4) and 3 x 10(‐3) M. 6. Various organic Ca2+ antagonists were effective in blocking the low‐threshold Ca2+ current in the following sequence: flunarizine greater than nicardipine greater than nifedipine greater than nimodipine greater than D600 (methoxyverapamil) greater than diltiazem. The corresponding KDs were 7 x 10(‐7), 3.5 x 10(‐6), 5 x 10(‐6), 7 x 10(‐6), 5 x 10(‐5) and 7 x 10(‐5) M. These Ca2+ antagonists induced a use‐dependent decrease in the current amplitude.(ABSTRACT TRUNCATED AT 400 WORDS)