Voltage‐gated sodium and calcium currents in rat osteoblasts.

Abstract

SUMMARY1. The whole-cell voltage-clamp mode of the patch-clamp technique was used to investigate the presence of voltage-gated inward currents in osteoblasts from newborn rat calvaria.2. In K+-free solutions, three kinds of inward currents could be activated by depolarization: a voltage-gated Na+ current and two different types of Ca21 currents.3. The Na+ current was activated by depolarization above -40 mV in all the cells. It was reduced by half by 10 nM-TTX (tetrodotoxin).4. In an isotonic Ba2+ external solution containing TTX, and with a Cs-EGTA internal solution buffered at pCa 8, depolarizing jumps induced both a transient Ba21 current and a sustained Ba21 current. The relative proportions of these two currents varied greatly among cells.5. The transient and sustained Ba2+ currents differ with respect to their time course and their voltage dependence.6. The depolarization-activated inward currents were also observed under more physiological conditions, in the presence of only 2 mM-external Ca2+ and with a K+ internal solution buffered at pCa 7.7. A few records obtained in current clamp showed that it is possible to induce action potentials in osteoblasts.

24R,25-(OH)2 Vitamin D3 Inhibits 1α,25-(OH)2 Vitamin D3 and Testosterone Potentiation of Calcium Channels in Osteosarcoma Cells

“…The rapid increase of intracellular calcium concentration caused by vitamin D 3 analogues and other steroid hormones is an area of increasing investigation (2-13, 29, 30). 1␣,25-D 3 and its analogues rapidly activate inward calcium movement in osteoblasts or osteoblast-like cells (2)(3)(4)(5)(6)(7)12). Using acetoxymethyl Quin2 fluorescent calcium indicator, Lieberherr (6) concluded, based on inhibition by dihydropyridine drugs like nitrendipine, that 70% of the increase in intracellular calcium concentration caused by 1␣,25-D 3 was from calcium influx via L-type calcium channels, suggesting this is a major pathway for calcium influx.…”

“…It has been documented by patch clamp recordings of whole-cell barium currents (2,3), by 45 Ca 2ϩ uptake (5,7), and by fluorescence measurements of intracellular calcium, using the calcium indicator acetoxymethyl Quin2 (10), that 1␣,25-D 3 (2) and other vitamin D 3 metabolites (3,5,7,8,(12)(13)(14) stimulate calcium influx into rat osteosarcoma cells (2,3,5,7,8,12,14) and human and rat osteoblasts (4,6) in primary culture. Guggino et al (15) and Caffrey and FarachCarson (2) have shown the presence of L-type calcium channels in ROS 17/2.8 osteoblast-like osteosarcoma cells.…”

“…A newly described function of 1␣,25-D 3 1 and its analogues is the rapid (in seconds) regulation of intracellular calcium concentration, first reported in chicken duodenum (1) and later in bone osteoblast cells (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). It has been documented by patch clamp recordings of whole-cell barium currents (2,3), by 45 Ca 2ϩ uptake (5,7), and by fluorescence measurements of intracellular calcium, using the calcium indicator acetoxymethyl Quin2 (10), that 1␣,25-D 3 (2) and other vitamin D 3 metabolites (3,5,7,8,(12)(13)(14) stimulate calcium influx into rat osteosarcoma cells (2,3,5,7,8,12,14) and human and rat osteoblasts (4,6) in primary culture.…”

Ribozyme Ablation Demonstrates That the Cardiac Subtype of the Voltage-sensitive Calcium Channel Is the Molecular Transducer of 1,25-Dihydroxyvitamin D3-stimulated Calcium Influx in Osteoblastic Cells

“…L-type VSCCs are the best studied Ca 2ϩ channel type and appear to be the major VSCC in osteoblasts, although T-type channels may be expressed transiently during development (36,37). Three types of cDNA sequences corresponding to ␣ 1S , ␣ 1C , and ␣ 1D were detected by Barry et al (8) in the rat UMR-106 osteosarcoma cell line.…”

A voltage-dependent calcium current in mouse MC3T3-E1 osteogenic cells.

“…That study revealed a receptor-operated Ca channel in the osteoblastic cell membrane. In addition, voltage-dependent Ca channels were demonstrated in the cell membrane of rat osteoblasts in culture (CHESNOY-MARCHAIS and FRITSCH, 1988). In both cases, a change in [Ca], due to Ca influx through the Ca channels is considered to be one of the early events of cell activation.…”