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O Sand

Publications and source records attributed to O Sand.

71 records · Page 4Linked to original sources
PubMed1981

Evidence for a function of calcium influx in the stimulation of hormone release fron the parathyroid gland in the goat.

The acute effects of various drugs on the release of parathyroid hormone (PTH) in goats were studied by local infusions in vivo. Infusions of Ca2+ or Sr2+ reduced the PTH secretion rate, whereas hypocalcemia induced by EDTA increased the PTH release. Blockers of voltage sensitive Ca2+ channels (verapamil, D-600 and nifedipine) lowered the PTH secretion rate, while infusion of 4-aminopyridine, which is a blocker of voltage sensitive K+ channels, increased the PTH release. These effects were not due to altered beta-adrenergic tonus, since the effects persisted when the drugs were administered during continuous infusion of the beta-blocker propranolol. We suggest that the parathyroid cells possess voltage sensitive K+ and Ca2+ channels, and that exocytosis of stored PTH depends on the influx of extracellular Ca2+ as in other secretory cells. In order to explain the inverse relationship between the plasma Ca2+ level and the PTH release, we postulate a suppressive effect of the plasma Ca2+ on the membrane permeability to Ca2+ in parathyroid cells.

4-Aminopyridine↗
PubMed1981

Electrophysiology of cultured parathyroid cells from the goat.

The membrane potential and resistance of goat parathyroid (PT) cells in culture were 44.7 +/- 6.8 mV and 503 +/- 218 M omega (mean +/- S.D., n = 24) in 2.4 mM Ca2+ solution. The corresponding values in 0.8 mM Ca2+ solution were 29.2 +/- 12.2 mV and 290 +/- 198 M omega (n = 22). The current/voltage relation showed a pronounced outward rectification for membrane potentials more positive than --40 mV. Current injection produced graded, regenerative responses in half of the penetrated cells. Regenerative responses were also obtained in Na+-free solution, and these responses were reversibly inhibited by the Ca2+ blocker D-600. The results support the notion that PT cells possess voltage sensitive K+ and Ca2+ channels, and that exocytosis of stored PT hormone depends on the influx of extracellular Ca2+ as in other secretory cells.

Animals↗
PubMed1980

Effects of thyroliberin and 4-aminopyridine on action potentials and prolactin release and synthesis in rat pituitary cells in culture.

Effects of thyroliberin (TRH) and 4-aminopyridine (4AP) were studied on prolactin (PRL) secreting rat pituitary tumour cells in culture (GH3 cells). Intracellular recordings obtained from the same cell before and during TRH stimulation showed this peptide to increase the spontaneous firing frequency and prolong the Ca2+ dependent action potentials. These effects were mimicked by 4 AP, which acts by interfering selectively with voltage dependent ionic channels without affecting resting membrane properties. Optimal doses of TRH and 4AP approximately doubled the release of PRL. In contrast, TRH increased PRL synthesis 1.9-fold while 4AP had no effect. Increased PRL synthesis is thus not a direct consequence of the hormone release. We conclude that TRH and 4AP both stimulates PRL release via the facilitating effects on the action potentials. TRH has additional intracellular effects which lead to increased synthesis of the hormone. The effects of TRH responsible for stimulation of PRL synthesis are not causally related to the impulse activity of the surface membrane of the cell.

Action Potentials↗
PubMed1979

Measurement of calcium ion concentrations in the lateral line cupulae of Xenopus laevis.

1. Ion selective microelectrodes were used to measure the distribution of Ca2+ in the cupulae of Xenopus laevis, and to observe the effects of the Ca2+ blocker, La3+, and a Ca2+ chelating agent (EGTA) on the endocupular potential and K+ concentration. 2. Measurements of the endocupular Ca2+ and potential were found to be in the range of 2-30 muM and 55-10 mV respectively, and their relationship indicates that Ca2+ is passively distributed in the cupula. 3. The concentration of endocupular K+ and endocupular potential was found not to be influenced by addition of either 1 mM EGTA or 100 muM-La3+ to the bathing solution.

Animals↗
PubMed1978

Electric activity of rat anterior pituitary cells in vitro.

Regenerative responses were seen in most cells either after cessation of an inward current pulse or during an outward current pulse. Two cell groups were distinguished electrophysiologically. Type I cells showed action potentials with maximum rate of rise of 21.2+/-9.0 V/sec (mean+/-S.D., n=19), while type II cells generated small graded depolarising responses with maximum rate of rise less than 3 V/s. The resting potentials of type I and II cells were 31.8+/-14.9 mV (n=19) and 41.7+/-9.8 mV (n=31), respectively. The steady-state current/voltage relationship was linear for both cell types when the membrane potential was more negative than -60 mV. An outward rectification appeared when the membrane potential was more positive than -40 mV. The input resistance was smaller in type I cells (274+/-212 Momega, n=19) than in type II cells (1 112+/-456 Momega, n=16). Even in Na-free solution regenerative responses were observed in most cells. When the Ca2+ concentration was increased tenfold to 24 mM, the maximum rate of rise of the off-response increased from 1.9+/-0.8 V/s (n=11) to 5.7+/-2.12 V/s (n=5). All-or-none action potentials could be evoked in this Ca2+ rich solution. Action potentials of similar maximum rate of rise could be evoked after replacing 24 mM Ca2+ with isomolar Sr2+. Prolonged action potentials were seen after subsitution of Ca2+ for Ba2+. It is concluded that action potentials in most anterior pituitary cells have a Ca component, which in type I cells is additional to a Na component.

Action Potentials↗
PubMed1975

Voltage clamp analysis of two inward current mechanisms in the egg cell membrane of a starfish.

Ionic mechanisms of excitation were studied in the immature egg cell membrane of a starfish, Mediaster aequalis, by analyzing membrane currents during voltage clamp. The cell membrane shows two different inward current mechanisms. One is activated at a membrane potential of -55 approximately -50 mV and the other at -7 approximately -6 mV. They are referred to as channels I and II, respectively. A similar difference is also found in the membrane potential of half inactivation. Currents of the two channels can, therefore, be separated by selective inactivation. The currents of both channels depend on Ca++ (Sr++ or Ba++) but only the current of channel I depends on Na+. The time-course of current differs significantly between the two channels when compared at the same membrane potential. The relationship between the membrane current and the concentration of the permeant ions is also different between the two channels. The result suggests that channel II is a more saturable system. The sensitivity of the current to blocking cations such as Co++ or Mg++ is substantially greater in channel II than in channel I. Currents of both channels depend on the external pH with an apparent pK of 5.6. They are insensitive to 3 muM tetrodotoxin (TTX) but are eliminated totally by 7.3 mM procaine. The properties of channel II are similar to those of the Ca channel found in various adult tissues. The properties of channel I differ, however, from those of either the typical Ca or Na channels. Although the current of the channel depends on the external Na the amplitude of the Na current decreases not only with the Na concentration but also with the Ca concentration. No selectivity is found among Li+, Na+, Rb+, and Cs+. The experimental result suggests that Na+ does not carry current but modifies the current carried by Ca in channel I.

Action Potentials↗