Electrical slow waves in the mouse oviduct are dependent on extracellular and intracellular calcium sources

Spontaneous contractions of the myosalpinx are critical for oocyte transport along the oviduct. Slow waves, the electrical events that underlie myosalpinx contractions, are generated by a specialized network of pacemaker cells called oviduct interstitial cells of Cajal (ICC-OVI). The ionic basis of oviduct pacemaker activity is unknown. Intracellular recordings and Ca ²⁺ imaging were performed to examine the role of extracellular and intracellular Ca ²⁺ sources in slow wave generation. RT-PCR was performed to determine the transcriptional expression of Ca ²⁺ channels. Molecular studies revealed most isoforms of L- and T-type calcium channels (Cav1.2,1.3,1.4,3.1,3.2,3.3) were expressed in myosalpinx. Reduction of extracellular Ca ²⁺ concentration ([Ca ²⁺]o) resulted in the abolition of slow waves and myosalpinx contractions without significantly affecting resting membrane potential (RMP). Spontaneous Ca ²⁺ waves spread through ICC-OVI cells at a similar frequency to slow waves and were inhibited by reduced [Ca ²⁺] _o. Nifedipine depolarized RMP and inhibited slow waves; however, pacemaker activity returned when the membrane was repolarized with reduced extracellular K ⁺ concentration ([K ⁺] _o). Ni ²⁺ also depolarized RMP but failed to block slow waves. The importance of ryanodine and inositol 1,4,5 trisphosphate-sensitive stores were examined using ryanodine, tetracaine, caffeine, and 2-aminoethyl diphenylborinate. Results suggest that although both stores are involved in regulation of slow wave frequency, neither are exclusively essential. The sarco/endoplasmic reticulum Ca ²⁺-ATPase (SERCA) pump inhibitor cyclopiazonic acid inhibited pacemaker activity and Ca ²⁺ waves suggesting that a functional SERCA pump is necessary for pacemaker activity. In conclusion, results from this study suggest that slow wave generation in the oviduct is voltage dependent, occurs in a membrane potential window, and is dependent on extracellular calcium and functional SERCA pumps.