Timing of exposure to somatostatin relative to growth hormone-releasing factor dictates the rat anterior pituitary cell growth hormone response.

The temporal interactions of rat GH-releasing factor (GRF) and somatostatin (SRIF) on the secretion of GH from perifused rat anterior pituitary cells have been studied. SRIF and GRF were employed at concentrations in a range close to levels reported in the hypophysial circulation of the rat. GH secretion was inhibited by pulses of 1 nmol SRIF/l (6 min) or 0.3 nmol SRIF/l (6 or 20 min). No rebound GH stimulation was observed. Exposure of cells to 6-min pulses of 0.3 nmol rat GRF/l repeated three times resulted in rapid stimulatory responses which became attenuated. Concomitant exposure to 0.3 nmol SRIF/l during a GRF pulse resulted in transient inhibition followed by a delayed and enhanced GH response, measured as the area under the curve but not peak height, whereas continuation of SRIF after a GRF pulse abolished the GH response. Exposure to 0.3 nmol SRIF/l prior to but not during a GRF pulse did not delay or inhibit the GH response. The area under the curve was increased under these conditions because of the lowered baseline in cells perifused with SRIF. SRIF alters somatotroph responsivity to equimolar concentrations of rat GRF and under different temporal conditions and can inhibit, enhance or delay GH secretion.