Dual effects of manganese on prolactin secretion

The effect of Mn2+ (a commonly used Ca2+ antagonist) on prolactin secretion from pituitary cells was investigated. In the presence of normal extracellular Ca2+ levels (2.5mM), Mn2+ inhibited basal, TRH- and K+- stimulated prolactin secretion. The Ca2+ ionophore, A23187, partially overcame the inhibitory effect of Mn2+. However, in the presence of low extracellular Ca2+ (less than 100 microM), which decreased basal prolactin secretion and abolished any stimulatory effects of TRH or K+, a paradoxical stimulatory effect was observed with Mn2+ in the presence of A23187. In the presence of Ca2+, Mn2+ appeared to be inhibitory due to its Ca2+ antagonistic effects, but at low Ca2+ levels, intracellular stimulatory effects of Mn2+ became apparent.