Voltage-gated sodium channels (NaCh) are important modulators of membrane potential and are essential for the action potential in nerve and muscle. At least two mammalian gene subfamilies exist, the first of which encodes the well characterized neuronal (brain types I-III), cardiac, and skeletal muscle isoforms. The second subfamily has only recently been discovered and contains one well characterized member (designated hNav2.1 in human and mNav2.3 in mouse) that is probably most commonly expressed in glial cells associated with peripheral nerve in heart, uterus, and lung. This channel is absent from the uterine myocyte surface in virgin and early pregnancy uterus. However, it is expressed at high levels in uterine smooth muscle at late pregnancy and then disappears from the myocyte cell surface within several days after delivery. This transient expression on the uterine myocyte surface strongly suggests that this protein plays either a delect or permissive role in uterine contraction at term. This proposal will further expand our understanding of the physiological role of the hNav2.1/mNav2.3 channels by 1) determining the cell specificity of protein expression in the fetal and adult mouse and human near term uterus, 2) determining the subunit composition of the channel in term uterus, 3) studying channel function using either heterologously expressed cDNA clones or uterine myocytes, and 4) characterizing the mouse Nav2.3 gene. Goals 1 and 4 are necessary prior to proceeding with a future gene deletion experiment in a transgenic mouse. These research efforts will further our understanding of a protein which is important in the regulation of uterine contractility. Knowledge gained through this study will provide the necessary background for the development of improved drugs for regulating uterine contraction, understanding uterine gene and protein regulation during pregnancy, and analysis of the physiological consequences of deleting the Nav2.3 gene from a transgenic mouse. In addition, due to the atypical amino sequence in functionally important regions of the 2.1/2.3 channels, this NaCh isoform promises to greatly increase our understanding of voltage-gated sodium channel structure function relationships.