Inactivation of the low-threshold transient calcium current in rat sensory neurones: evidence for a dual process

In rat cranial sensory neurones a transient Ca current (iCa,t) is elicited by depolarizing the membrane potential from -80 mV to beyond -50 mV. In this paper the characteristics of the slow and fast inactivation processes of this current are described. Recordings were obtained in whole-cell clamp conditions from Cs-loaded cells. For most experiments, cells were dialysed at an internal pCa of 8, and Na and K currents were eliminated using a choline chloride- and K-free external medium containing 5 mM-Ca and 2 mM-Mg. The decay of iCa,t could be approximately fitted by a single exponential with a voltage-dependent time constant which decreased from about 150 ms at -50 mV to about 25 ms at -20 mV. This suggests a single process of inactivation but a detailed kinetic analysis of the onset and the offset of the inactivation revealed biphasic processes. The onset of inactivation displays two exponential phases. The fast phase lasts for 100-500 ms, and the slow phase lasts for a few seconds. The relative amplitude and the time constants of each phase vary with the inactivating potential. The recovery from inactivation is also biphasic, with either a fast or a slow component predominating, depending on whether a short- (some hundreds of milli-seconds) or a long- (in the order of tens of seconds) inactivating pulse has been used. At -80 mV, after a 300 ms inactivating pulse, responses recover to at least 40% of maximum within 200 ms and recovery is complete within 1 s; after a long predepolarization (10-20 s), recovery takes 4-5 s. Fast recovery was observed best after large but brief depolarizations and slow recovery was observed best following long inactivating pre-pulses of small amplitude. The voltage-dependence of slow and fast inactivation was determined by realizing inactivation curves. Fast inactivation developed between -60 and -20 mV while the slow process occurred at more hyperpolarized potentials, e.g. at -75 to -50 mV. Fast inactivation was not altered by the entry of Ca during the previous activation of the channel. Further, decay of iCa,t was not modified when Ba was substituted for Ca or the internal pCa was decreased. These are indications of a uniquely voltage-dependent process. A possible role of Ca entry in slow inactivation is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)