okadaic-acid and 8-bromoadenosine-3--5--cyclic-monophosphorothioate

A slowly activating, outwardly rectifying Cl- channel (ORCC) has been described in rat peritoneal mast cells (RPMCs). This channel is activated by intracellular application of cAMP, an effect that might be mediated by a PKA-type serine/threonine protein kinase. To test this hypothesis, whole-cell patch-clamp experiments (nystatin-perforated patch) were performed and 8-bromoadenosine 3',5'-cyclic monophosphothioate, Sp-enantiomer (Sp-8Br-cAMPS), a cell membrane-permeable activator of PKA, and three inhibitors of different serine/threonine protein phosphatases (okadaic acid, cantharidin, calyculin A), were tested. In RPMCs application of repetitive series of step hyper- and depolarizations (holding potential 0 mV, test potentials -80 to +80 mV, step size +20 mV) induced a slowly increasing, [half-maximal activation time ( t0.5) 11.0+/-1.1 min, Imax (at +80 mV) 18.7+/-3.1 pA pF-1], DIDS-sensitive, outwardly rectifying Cl- current I(Cl,OR). The activation of this current could be accelerated by Sp-8Br-cAMPS, okadaic acid or cantharidin in the extracellular solution. Co-application of Sp-8Br-cAMPS and okadaic acid increased Imax supra-additively. Calyculin A and higher concentrations of cantharidin inhibited the Cl- current via unknown mechanisms. Our findings suggest that I(Cl,OR) in RPMCs is activated by a PKA-type protein kinase, a process which is antagonized functionally by okadaic acid- and cantharidin-sensitive protein phosphatases.

Topics: 8-Bromo Cyclic Adenosine Monophosphate; Animals; Chloride Channels; Chlorides; Cyclic AMP-Dependent Protein Kinases; Enzyme Inhibitors; Male; Marine Toxins; Mast Cells; Membrane Potentials; Okadaic Acid; Oxazoles; Patch-Clamp Techniques; Peritoneal Cavity; Phosphoric Monoester Hydrolases; Protein Serine-Threonine Kinases; Rats; Rats, Wistar; Thionucleotides

The effect of the physiological vitamin D metabolite 24R, 25-dihydroxyvitamin D3 [24R,25(OH)2D3] on human osteoblastic cells was assessed. Physiological concentrations (10(-9)-10(-8) M) of 24R, 25(OH)2D3 significantly increased the cyclic guanosine 5'-monophosphate (cGMP) content in the human osteoblastic cells by approximately 200% in 5 to 15 min. In contrast, 24S, 25-dihydroxyvitamin D3 had only a weak effect on the cGMP content, and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] did not affect the content. The production of osteocalcin was not induced by 10(-9)-10(-8) M of 24R,25(OH)2D3 in the absence of 1,25(OH)2D3. However, the same concentration of 24R,25(OH)2D3 showed stimulatory effects on osteocalcin synthesis in the presence of 10(-9) M 1, 25(OH)2D3. Rp-8Br-cyclic GMP, a specific inhibitor of cyclic GMP-dependent protein kinase, significantly inhibited the cooperative effect of 24R,25(OH)2D3 with 1,25(OH)2D3 on the osteocalcin synthesis, although Rp-8Br-cyclic AMP, a specific inhibitor of cyclic AMP-dependent protein kinase, did not affect the cooperative effect. In addition, okadaic acid enhanced the osteocalcin synthesis induced by 1,25(OH)2D3. These observations suggest that 24R,25(OH)2D3 has a unique activity of increasing cGMP contents in osteoblastic cells, and that the increase in cGMP contents may lead to the cooperative effect of 24R,25(OH)2D3 with 1, 25(OH)2D3 on osteocalcin synthesis. These data support the hypothesis that 24R,25(OH)2D3 has a physiological role in human bone and mineral metabolism.

Topics: 24,25-Dihydroxyvitamin D 3; 8-Bromo Cyclic Adenosine Monophosphate; Calcitriol; Cells, Cultured; Cyclic GMP; Drug Synergism; Humans; Okadaic Acid; Osteoblasts; Osteocalcin; Thionucleotides