Characterization of the molecular and electrophysiological properties of the T-type calcium channel in human myometrium.

Rises in intracellular calcium are essential for contraction of human myometrial smooth muscle (HMSM) and hence parturition. The T-type calcium channel may play a role in this process. The aim was to investigate the role of the T-type calcium channel in HMSM by characterizing mRNA expression, protein localization, electrophysiological properties and function of the channel subunits Cav3.1(alpha1G), Cav3.2(alpha1H), and Cav3.3(alpha1I). QRT-PCR, immunohistochemistry, electrophysiology and invitro contractility were performed on human myometrial samples from term, preterm, labour and not in labour. QRT-PCR analysis of Cav3.1, Cav3.2 and Cav3.3 demonstrated expression of Cav3.1 and Cav3.2 with no significant change (P>0.05) associated with gestation or labour status. Immunohistochemistry localized Cav3.1 to myometrial and vascular smooth muscle cells whilst Cav3.2 localized to vascular endothelial cells and invading leucocytes. Voltage clamp studies demonstrated a T-type current in 55% of cells. Nickel block of T-type current was voltage sensitive (IC50 of 118.57+/-68.9 microM at -30 mV). Activation and inactivation curves of ICa currents in cells expressing T-type channels overlapped demonstrating steady state window currents at the resting membrane potential of myometrium at term. Current clamp analysis demonstrated that hyperpolarizing pulses to a membrane potential greater than -80 mV elicited rebound calcium spikes that were blocked reversibly by 100 microM nickel. Contractility studies demonstrated a reversible decrease in contraction frequency during application of 100 microM nickel (P<0.05). We conclude that the primary T-type subunit expressed in some MSMCs is Cav3.1. We found that application of 100 microM nickel to spontaneously contracting human myometrium reversibly slows contraction frequency.