Hiroyuki Soeda

Protein tyrosine kinase inhibitors increase cytosolic calcium and inhibit actin organization as resorbing activity in rat osteoclasts

Although there is evidence that protein tyrosine kinase inhibitors (PTKIs) suppress bone resorption activity, the mechanism of action of these compounds on osteoclastic bone resorption remains obscure. In the present study, we investigated the effect of PTKIs on cytosolic Ca2+ concentration ([Ca2+]i) and on the cytoskeleton in rat osteoclasts. The PTKIs, genistein and herbimycin A, reversibly elevated [Ca2+]i measured by fura‐2 microfluorimetry. The PTKI‐induced increase was abolished by omission of extracellular Ca2+, but was not attenuated by depletion of Ca2+ stores. The PTKI‐induced increase was inhibited by addition of La3+ and Ni2+, but not abolished by dihydropyridine (DHP) Ca2+ channel blockers. Genistin, an inactive analogue of genistein, had no effect on [Ca2+]i. In the cytoskeleton assay, genistein rapidly disrupted the actin ring formation that serves as a marker for the resorbing state of osteoclasts. Disruption of the actin ring formation was also diminished in Ca2+‐free extracellular solution. These results suggest that PTKIs in rat osteoclasts elevate [Ca2+]i via activation of a DHP‐insensitive, nonspecific Ca2+ entry pathway and disrupt the formation of actin rings, resulting in suppression of bone resorption activity. The regulation of this Ca2+‐influx by PTKIs is likely to contribute to inhibition of bone resorption by these compounds. J. Cell. Physiol. 183:83–90, 2000.

Estradiol inhibits Ca2+ and K+ channels in smooth muscle cells from pregnant rat myometrium

The purpose of this study was to investigate the actions of 17beta-estradiol on the electrical activity of pregnant rat myometrium. The longitudinal layer of the myometrium was dissected from pregnant rats (17 to 19 days of gestation), and single cells were isolated by enzymatic digestion. Calcium currents and potassium currents were recorded by the whole-cell voltage-clamp method, and the single calcium-dependent potassium current was recorded by the outside-out patch-clamp method. The effects of 17beta-estradiol on these currents were investigated. When a myometrial cell was held at -50 mV, depolarization to a potential more positive than -30 mV produced an inward current followed by a slowly developing outward current. Application of tetraethylammonium inhibited the outward current while the inward current was completely abolished in a calcium-free solution. Estradiol at high concentrations (> 3 microM) inhibited both inward and outward currents in a voltage-dependent manner. Removal of estradiol restored the amplitude of the outward but not of the inward current. Estradiol (30 microM) also inhibited the activity of single calcium-dependent potassium channels without changing single channel conductance. In conclusion, estradiol at high concentrations inhibited: (1) voltage-dependent calcium, (2) calcium-dependent potassium and (3) voltage-dependent potassium currents. These actions of estradiol would prevent action potential generation and after-hyperpolarizations. Suppression of the after-hyperpolarization might further prevent spike generation due to slowing of the calcium channels recovery from the inactivated state.

Electrical response to acid across the dorsal epithelium of the frog tongue

Acid stimulation produced a change in trans-epithelial potential difference across isolated dorsal epithelium of frog tongue. The responses to acetic acid, tartaric acid and citric acid were significantly greater than those to hydrochloric acid and nitric acid at the same pH. Metabolic inhibitors had no effect on the response, but local anaesthetics reversibly depressed it. The induced response was associated with a decrease in tissue resistance. Positive polarization on the mucosa increased the response, which was decreased or reversed under negative polarization. These responses may be caused by diffusion of H-ions through the tissue, and may influence sour reception in the frog.

Epithelial responses to glycinamide and glycylglycinamide in the frog (Rana catesbeiana) tongue

1. Open-circuit potential difference and short-circuit current across the frog tongue epithelium in response to glycinamide and glycylglycinamide were investigated. 2. Response to both of these amides were larger than the responses produced by glycine, glycylglycine and NaCl, and were independent of Na+ in the mucosal medium but dependent on H+ in the stimulus. 3. The relationships of the magnitudes of both response to the stimulus were similar to that in the taste nerve. 4. The results indicate that H+-dependent transport of these amides is related to taste reception in frogs.

Sugar-induced transepithelial responses across the tongue epithelium of the frog Rana catesbeiana

Abstract Changes in transepithelial potential difference were induced across the dorsal tongue epithelium of frogs ( Rana catesbeiana ) by stimulation with various sugars. These were observed as a positive phase followed by a negative phase in the submucosa. The characteristics of the positive phase differed in many respects from those of the negative phase, and its profile resembled the activity of the gustatory nerve. The response was composed of a positive component superimposed on and masked by a negative component. In conclusion, the tissue absorbed sugar coupled with sodium and induced a positive component, and this may be related to gustatory transduction in frogs.

Salt-induced electrical epithelial responses of the frog (Rana catesbeiana) tongue and their relation to gustatory nerve activity in vivo.

Stimulation of the frog tongue with various salts produced changes in epithelial potential of the tongue, accompanied by changes in gustatory nerve activity. Both changes varied similarly according to the stimulus. The results indicate that cation transport in the lingual epithelium is involved in gustation.

Electrical response to hypotonic NaCl Ringer solution across the dorsal epithelium of the tongue of the frog, Rana catesbeiana

Stimulation with hypotonic NaCl Ringer solution produced a change in potential across the epithelium, which was not caused by decreased osmotic pressure. The potential profile resembled the receptor potential of frog taste cells and so may be related to water reception.