Somatostatin inhibits cAMP-dependent and cAMP-independent calcium influx in the clonal pituitary tumor cell line AtT-20 through the same receptor population.

The characteristics of somatostatin (SRIF) inhibition of calcium influx stimulated by corticotropin releasing factor (CRF), an activator of adenylate cyclase, and K+, a membrane depolarizing agent, in AtT-20 cells were assessed. Changes in cytosolic calcium levels were measured using the fluorescence probe Quin 2. Both CRF and K+ raise cytosolic calcium levels by stimulating calcium influx. SRIF induced an immediate inhibition of CRF and K+-stimulated calcium influx. This effect was concentration-dependent with IC50 values for SRIF's blockade of CRF and K+ stimulation of 64 +/- 13 pM and 100 +/- 15 pM, respectively. The SRIF analogs, SRIF 28, Trp8-SRIF and Tyr11-SRIF had the same rank order of potency to block CRF and K+-induced calcium influx. The inhibitory effects of SRIF on AtT-20 cells were abolished by pertussis toxin pretreatment. SRIF inhibition of both CRF and K+-induced calcium influx desensitized. The desensitization was rapid (T1/2 approximately 2.5 min), dependent on the concentration of SRIF and not due to the degradation of the peptide. The ability of SRIF to block CRF (cyclic AMP-dependent) and K+ (cyclic AMP-independent)-stimulated calcium influx into AtT-20 cells cannot be separated.