Effects of Gadolinium and Cadmium on the Electrically Evoked Release of 45calcium from the Isolated Rat Neurohypophysis

Isolated neural lobes of the rat pituitary gland were fixed by their stalks to a platinum wire electrode. They were loaded with 45calcium and then superfused with radioactivity-free Krebs-solution. The efflux of 45calcium into the superfusion medium was determined. After 54-60 min of superfusion the spontaneous outflow of 45calcium was 2.03%/min of the tissue 45calcium. It was not affected by cadmium (Cd2+, 0.03-3 mmol/l), but reduced by 40% in the presence of 1 mmol/l gadolinium (Gd3+). Electrical stimulation with pulses of 15 Hz (3 times for 1 min with intervals of 1 min) evoked a 45calcium release of 14.4% of the tissue radioactivity. The evoked release of 45calcium was reduced by 80% in the presence of tetrodotoxin and by about 50% in the presence of gallopamil (D600, 30 mumol/l) or after omission of unlabelled calcium from the superfusion medium. Gd3+ concentration-dependently reduced the evoked release by maximally 75% at 3 mmol/l. However, it inhibited the evoked release of 45calcium less effectively than the release of vasopressin evoked by identical stimulation conditions. Cd2+ reduced the evoked release by maximally 55% at 300 mumol/l. The effect of Cd2+ on the evoked release of vasopressin was not tested because Cd2+ markedly increased the spontaneous outflow of vasopressin. When the stimulation was carried out for only 1 min at 15 Hz (i.e. 900 pulses) the evoked release of 45calcium was 10.6% of the tissue 45calcium and 100 mumol/l Cd2+ or 300 mumol/l Gd3+ caused a reduction of the evoked release similar to that observed when 3 periods of stimulation were applied.(ABSTRACT TRUNCATED AT 250 WORDS)