Introduction: Oviducts, or fallopian tubes (in primates), are smooth muscle-lined tubular organs that facilitate several essential physiological processes including gamete transport, ovum fertilization and early embryo development. Contractions of the oviduct smooth muscle (myosalpinx) and the wafting motion of the ciliated epithelium that lines these tubes, facilitate bi-directional transport of gametes so that the newly released ovum(s) is transported in one direction (pro-uterus) while spermatozoa are transported in the opposite direction (pro-ovary). For successful fertilization to occur, these transport processes must be temporally coordinated so that the ovum and spermatozoa meet each other in the ampulla; the site of fertilization. Once the ovum is fertilized, the early embryo then begins another precisely timed journey toward the uterus for implantation. Myosalpinx contractions also facilitate this journey while luminal secretions from secretory epithelial cells aid early embryo maturation and influence gamete viability.

Aims/Objectives: To develop a better understanding of the excitable nature of the oviduct myosalpinx, we utilized a mouse model with a genetically engineered calcium (Ca²⁺) indicator (PDGFRa/cre-Gcamp6f/loxp) that was expressed in the myosalpinx. Specific questions to be addressed were: (i) are all regions of the oviduct excitable? (ii) how does propagation occur along the different segments of the oviduct? (iii) did activity that originated in the oviduct propagate to the adjacent uterus? (iv) what were the cellular mechanisms responsible for the generation and propagation of Ca²⁺calcium waves in the oviduct? 

Methods: All animals were maintained, and the experiments performed in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals. 

Ca²⁺ imaging and spatiotemporal mapping was performed at different sites along the oviduct. Simultaneous intracellular microelectrode recordings and Ca²⁺ imaging was also performed. Confocal imaging was used to determine cellular expression of Gcamp6. 

Results: PDGFRa/cre-Gcamp6f/loxp mice displayed robust spontaneous Ca²⁺ waves throughout all regions of the oviduct. Ca²⁺ waves propagated from ovary to uterus, but also from uterus to oviduct, often colliding in a specialized region within the Ampulla/isthmus, the possible site of fertilization. Ca²⁺ waves that originated in the isthmus often propagated into the uterus activating an excitable wavefront that spread along this organ. Electrical slow waves were responsible for the generation of Ca²⁺ waves. Ca²⁺ waves were sensitive to removal of extracellular Ca²⁺, inhibition of intracellular Ca²⁺ stores with cyclopiazonic acid (CPA) and caffeine and were reduced by the Ano1 Ca²⁺-activated chloride channel (CaCC) inhibitor CaCC_inh‐A01. 

Conclusions: For the first time, we are able to track the generation and propagation of Ca²⁺ waves as an indicator of oviduct myosalpinx excitability. Propagation of Ca²⁺ waves that occurred in both directions along the oviduct suggests that myosalpinx activity contributes to the transport of both gametes. These events relied on the release of intracellular Ca²⁺ and were likely generated by the CaCC Ano1.