Role of intracellular Ca(2+)-stores in the 5-hydroxytryptamine-induced Ca(2+)-current inhibition in NG108-15 hybrid cells

Inhibition of voltage-dependent Ca(2+)-current by alpha-adrenoceptor agonists in smooth muscle cells

The cellular mechanisms underlying the inhibitory effects of phenylephrine on dihydropyridine-sensitive, voltage-dependent Ca2+ currents recorded from single smooth muscle cells dissociated from the rat anococcygeus muscle were examined. Phenylephrine (0.1-30 microM) produced a concentration-dependent inhibition of the Ca2+ current; the maximum response occurred at a concentration of 10 microM, which inhibited the peak inward current evoked at 0 mV by 57.7 +/- 4% (n = 8). The response to phenylephrine was reduced but not abolished in cells containing 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA; 10 mM), and it persisted in cells dialysed internally with heparin (5 mg.ml-1). This was despite the fact that both EGTA (5 mM) and heparin were able to block the phenylephrine-induced, Ca(2+)-dependent chloride current recorded in the same cells. The inhibition of the Ca2+ current produced by phenylephrine was abolished in cells containing guanosine 5'-[beta-thio]diphosphate (GDP-beta-S) but persisted in cells pre-treated with pertussis toxin. Our results suggest that the inhibition of L-type Ca2+ current seen following alpha-adrenoceptor activation occurs by a mechanism independent from the inositol trisphosphate-mediated release of Ca2+ from intracellular stores.

Serotonin-induced changes in membrane potential and cytosolic free calcium in a clonal pituitary cell line (AtT-20) of cultured mouse corticotropes

The effects of serotonin (5-HT) were examined on the cytosolic free calcium concentration, [Ca2+]i, in fura-2-loaded cells from a clonal cell line derived from the mouse anterior pituitary gland (AtT-20/D16v). Brief applications of 5-HT produced transient increases in [Ca2+]i. The duration and the amplitude of the 5-HT-induced increase in [Ca2+]i depended on the duration of 5-HT application. Longer duration (> 150 msec) applications of 5-HT produced a transient increase followed by a prolonged plateau phase of [Ca2+]i. Oscillations in [Ca2+]i were initiated and maintained in previously quiescent corticotropes following brief exposures to 5-HT. Addition of cobalt (500 microM-3.6 mM) to the extracellular solution abolished oscillations in [Ca2+]i and produced transient but not sustained increases in [Ca2+]i in response to 5-HT. Electrophysiological responses to 5-HT were recorded in separate experiments using the whole-cell patch clamp recording technique. Application of 5-HT to single cells produced a depolarization which was accompanied by a decrease in membrane conductance when measured under voltage clamp. The duration of the depolarizing response also increased with the duration of the 5-HT application. With longer duration 5-HT applications, the 5-HT-induced depolarizing response resembled the spontaneous action potential responses recorded in the same cell. However, the 5-HT-induced depolarizations were unaltered in the presence of extracellular cobalt. This suggests that the transient increase in [Ca2+]i recorded when 5-HT was applied in the presence of cobalt may represent intracellular release of calcium. These results demonstrate that 5-HT activates single cultured corticotropes directly by producing an increase in [Ca2+]i and cell depolarization.