K+ and Ca2+ Channels Regulate Ca2+ Signaling in Chondrocytes: An Illustrated Review

An improved understanding of fundamental physiological principles and progressive pathophysiological processes in human articular joints (e.g., shoulders, knees, elbows) requires detailed investigations of two principal cell types: synovial fibroblasts and chondrocytes. Our studies, done in the past 8–10 years, have used electrophysiological, Ca2+ imaging, single molecule monitoring, immunocytochemical, and molecular methods to investigate regulation of the resting membrane potential (ER) and intracellular Ca2+ levels in human chondrocytes maintained in 2-D culture. Insights from these published papers are as follows: (1) Chondrocyte preparations express a number of different ion channels that can regulate their ER. (2) Understanding the basis for ER requires knowledge of a) the presence or absence of ligand (ATP/histamine) stimulation and b) the extraordinary ionic composition and ionic strength of synovial fluid. (3) In our chondrocyte preparations, at least two types of Ca2+-activated K+ channels are expressed and can significantly hyperpolarize ER. (4) Accounting for changes in ER can provide insights into the functional roles of the ligand-dependent Ca2+ influx through store-operated Ca2+ channels. Some of the findings are illustrated in this review. Our summary diagram suggests that, in chondrocytes, the K+ and Ca2+ channels are linked in a positive feedback loop that can augment Ca2+ influx and therefore regulate lubricant and cytokine secretion and gene transcription.