Comparative studies of membrane potentials of rat osteoblasts in situ and in primary culture.

The aim of the present study was to compare the membrane potential, Vm, of rat calvarium osteoblasts in situ and during primary culture using electrophysiological techniques. For osteoblasts studied in situ the Vm (mean +/- S.D.) was found to be -42.0 +/- 10 mV, n = 32. Comparable values (-46.8 +/- 9.n, n = 152) were found for cultured osteoblasts during the first week. Thereafter when cells began to form multilayers an increase in Vm negativity was observed (-58.6 +/- 12.8, n = 82). The Vm of osteoblasts in situ and during the first week of culture were highly dependent on the Na+ diffusion potential and slightly influenced by the increase in extracellular [K+]. In contrast, the more negative Vm of the cells cultured 2 weeks were highly dependent on the K+ diffusion potential and were not influenced by changes in external [Na+]. Parathyroid hormone (bPTH(1-34], 240 nM, in the external solution, induced a rapid membrane depolarization in situ (+10.2 +/- 5.0, n = 18) as it does in cultured cells. Additional investigation on the possible effects of Ca2+ and PO4(2-) (since, in situ, mineralized matrix prevents such an approach) showed that low external [Ca2+] only slightly hyperpolarizes the membrane, whereas cell Vm was highly sensitive to changes in external [PO4(2-)].