Figure 7.

Wm-induced inhibition of PRL release occurs downstream of spontaneous and augmented voltage-gated calcium influx. (A) Bay K8644 (BayK), an L-type calcium channel agonist, induced rise in cytosolic calcium concentration ([Ca²⁺]_i), which was not affected by treatment of cells with 10 µM Wm for 45 min. (B) Facilitation of voltage-gated calcium influx by high K⁺-induced depolarization of cells was also not affected by treating the cells with 10 µM Wm for 45 min. Horizontal red bars indicate the duration of BayK and high K⁺ treatments, and the arrows indicate the time of application of thyrotropin-releasing hormone (TRH) in the absence or presence of Wm. In addition to TRH, cells were stimulated with 1 µM dopamine, to identify lactotrophs (not shown). Traces shown are representative from three independent experiments. (C, D) Although Wm pretreatment did not affect the facilitated voltage-gated calcium influx, the stimulatory effect of BayK (C) and high K⁺ (D) on PRL release was dramatically reduced in pituitary cells perfused with Wm-containing medium for 45 minutes. The mean ± SEM values for PRL release (ng/ml) during the first 20 min of BayK treatments in the absence and presence of Wm in four experiments were: 210 ± 13 vs. 42 ± 2.4; 146 ± 10 vs. 31 ± 1; 184 ± 11 vs. 35 ± 2; and 481 ± 24 vs. 193 ± 11. The mean ± SEM values for two high potassium experiments in the absence and presence of Wm were 173 ± 21 vs. 18 ± 1; and 255 ± 172 vs. 5 ± 3. In all cases, P < 0.0001.