MODULATION OF IONIC CHANNELS IN CULTURED OSTEOBLASTS

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Bone is a complex living tissue containing osteoblasts and osteoclasts which are primarily responsible for bone formation and resorption. Recently, a number of voltage-gated channels have been found to be present in the cell membranes of osteoblasts. These channels which allow ion movements across the membrane down their electrochemical gradients are traditionally found in excitable membranes such as neurons, various types of muscle cells as well as in secretory cells. Osteoblasts secrete a number of growth factors and thus may have a variety of ionic channels that are also present in other secretory cells. External factors such as hormones, drugs and mechanical forces are known to modulate the physiological functions of bone cells. Parathyroid hormone, for example, is known to first depolarize the membrane of osteoblasts, which is then followed, several minutes later, by membrane hyperpolarization. The mechanism of action of parathyroid hormone in inducing these membrane potential changes remains largely unknown. However, variations in the permeability of ionic channels can easily account for the changes observed. The depolarization caused by the entry of cations into a cell can also trigger the release of second messengers, which in turn, can modulate the properties of ionic channels. Mechanical forces also play an important role in the differentiation and growth of the skeleton, with increased activity resulting in bone growth while a decrease in activity would lead to mineral loss. This would suggest the presence of receptors in bone cells that are sensitive to stretch or tension. Indeed, stretch-activated ionic channels have been found in a variety of cell types. It is the objective of this proposal to utilize varying aspects of the patch-clamp technique, which is uniquely suitable for studying the properties of ionic channels, to determine how these channels are modulated by external factors using cultured osteoblasts dissociated enzymatically from neonatal rat calvaria. The properties of the ionic channels found in these osteoblasts will then be compared to those that have been reported in other traditionally excitable membranes.