ABSTRACT
Smooth muscle cells constitute a heterogeneous collection of effector cells that, by virtue of both their constituency in blood vessels and presence as primary parenchymal cells in diverse tissues, affect the function of all organs. Thus, perhaps it is not surprising that alterations in, and/or dysfunction of, smooth muscle cells are quite common, and responsible, at least in part, for the morbidity and mortality associated with a very wide range of human diseases. These facts point to the necessity for improved understanding of the mechanism(s) governing the control of myocyte contractility (i.e., tone). Such understanding has been rapidly forthcoming in recent years, and has indicated that in many smooth muscle cell types intercellular communication through gap junctions acts in concert with nonjunctional (K+) ion channels to make important contributions to the control of myocyte tone and tissue homeostasis in physiologically diverse organs. Intercellular communication through connexin43-derived gap junction channels and K+ flux through the KCa and KATP channel subtypes, in particular, appear to play prominent roles in this process. The goal of this report, therefore, is to review the data concerning junctional and nonjunctional ion channels on the detrusor myocytes of the urinary bladder, as well as on the specialized vascular myocytes of the corpus cavernosum. The choice of an excitable (i.e., bladder detrusor myocytes) and nonexcitable (i.e., corporal smooth muscle) smooth muscle cell type ensures that the discussion will at least encompass consideration of a large portion of the spectrum of physiological possibilities for the participation of junctional and nonjunctional ion channels in the initiation, maintenance and modulation of smooth muscle tone. A central thesis of this communication is that detailed knowledge of the myocyte- and tissue-specific properties of K+ channels and gap junctions will likely lead to the improved understanding and treatment of human smooth muscle diseases/disorders.
