Molecular and electrophysiological studies of neuronal A-type and M-type K(+) channels
Description
Cloned A-channel candidates (Kv1.4, 3.4, 4.1, 4.2 and 4.3) were initially expressed in Xenopus oocytes. However, functional discrepancies exist between native A-currents and currents resulting from Kv alpha-subunit expression in oocytes. The discrepancies may stem from heteromeric subunit assembly or may simply result from the different expression environments. Therefore, we compared native A-current (IA) from adult rat sympathetic neurons with those from cloned A-channel candidates expressed in HEK293T cells. Native and cloned IA were recorded under similar experimental conditions using the patch-clamp technique. IA of SCG neurons activated at more hyperpolarized voltages than cloned IA. Inactivation of native IA occurred at more negative potentials and the recovery from inactivation was more rapid than that of cloned IA. Native current was more sensitive to 4-aminopyridine than currents from the Kv 4 family. These results demonstrate that single cloned Kv-channel alpha-subunits do not duplicate the native A-current of sympathetic neurons M-channels can be modulated by a large array of Gq/11-coupled receptors. The histamine H1 receptor (H1R) also appears to be a Gq/11-coupled receptor, however, the involvement of H 1R in M-channel modulation has not been addressed. Therefore, we investigated if histamine could inhibit recombinant M-channels produced by KCNQ2/KCNQ3 channel subunit expression via H1R receptors expressed in HEK 293T and HeLa cells. Our studies showed that activation of H1R by histamine significantly inhibited recombinant M-currents (IM). This inhibition was not a peculiarity of the HEK 293T cell since IM inhibition by histamine also occurred in HeLa cells. Activation of a pertussis toxin-insensitive G protein was involved in histamine inhibition. Moreover, Galphaq/11 is a mediator of the inhibition since expression of the Galpha q/11 buffer significantly prevented histamine inhibition. However, expression of Gbetagamma buffers also strongly attenuated IM inhibition by histamine, indicating that the Gbetagamma dimer was also involved in IM modulation by histamine. The histamine inhibition of I M was not mediated by Ca2+ release from an intracellular Ca2+ store since thapsigargin pretreatment with could not prevent histamine inhibition. Pretreatment with nonspecific protein kinases inhibitors were also unable to prevent histamine inhibition, indicating activation of protein kinases was not involved in histamine inhibition of IM