Electrophysiological characteristics were compared among four cloned human alpha1H isoforms transcripted by alternative splicings of exons 25B and 26 [Delta25B/+26 (native form; alpha1H-a), Delta25B/Delta?6 (alpha1H-b), +25B/Delta26, and +25B/+26] in the intracellular loop between domains III and IV (III-IV linker) of a human T-type calcium channel (Ca(v)3.2). The native isoform Delta25B/+26 predominated in ovary and non-pregnant uterus, while isoform Delta25B/Delta26 (alpha1H-b) predominated in pregnant uterus and testis. Expressions of the newly identified +25B/Delta26 and +25B/+26 isoforms were greater in the uterus at gestation than in the non-pregnant uterus. When expressed in Xenopus laevis oocytes, all isoforms produced transient inward currents with low voltage-dependent activation and inactivation characterized in typical T-type Ca2+ currents. Each isoform possessing exon 25B (+25B/?Delta26 or +25B/+26) showed current activation and inactivation at a more negative membrane potential than the respective isoform (Delta25B/Delta26 or Delta25B/+26) lacking it. Moreover, the current activation and inactivation rates were faster for the two isoforms possessing exon 25B than for the respective isoforms lacking it. By itself, exon 26 seemed not to affect any electrophysiological characteristics. Increasing the net positive charge (relative to the native form), as occurred in isoforms Delta25B/Delta26, +25B/Delta26, and +25B/+26, caused recovery from short-term inactivation to become faster. Our results show that molecular-structure variations within the III-IV linker influence the voltage-dependence and kinetics of both activation and inactivation. Although the role of T-type Ca2+ channels in uterine tissue remains unknown, changes in the uterine expression of these alpha1H isoforms may influence physiological functions during pregnancy.
(c) 2005 S. Karger AG, Basel