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 Cav 3.1(α1G), Cav 3.2(α1H), and Cav 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 Cav 3.1, Cav 3.2 and Cav 3.3 demonstrated expression of Cav 3.1 and Cav 3.2 with no significant change (P > 0.05) associated with gestation or labour status. Immunohistochemistry localized Cav 3.1 to myometrial and vascular smooth muscle cells whilst Cav 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 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 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 Cav 3.1. We found that application of 100 μM nickel to spontaneously contracting human myometrium reversibly slows contraction frequency.
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