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 Ca_v 3.1(α1G), Ca_v 3.2(α1H), and Ca_v 3.3(α1I). 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 Ca_v 3.1, Ca_v 3.2 and Ca_v 3.3 demonstrated expression of Ca_v 3.1 and Ca_v 3.2 with no significant change (P > 0.05) associated with gestation or labour status. Immunohistochemistry localized Ca_v 3.1 to myometrial and vascular smooth muscle cells whilst Ca_v 3.2 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 μM at -30 mV). Activation and inactivation curves of I_Ca 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 μM nickel. Contractility studies demonstrated a reversible decrease in contraction frequency during application of 100 μM nickel (P < 0.05). We conclude that the primary T-type subunit expressed in some MSMCs is Ca_v 3.1. We found that application of 100 μM nickel to spontaneously contracting human myometrium reversibly slows contraction frequency.