Somatostatin blocks a calcium current in rat sympathetic ganglion neurones.

Abstract

1. The effects of somatostatin and somatostatin analogues on a Ca2+ current from acutely isolated and short-term (24-48 h) cultured adult rat superior cervical ganglion (SCG) neurones were studied using the whole-cell variant of the patchclamp technique. 2. [D-Trp8]Somatostatin (SOM) produced a rapid, reversible and concentration-dependent reduction of the CA2+ current. CA2+ current amplitude was reduced over the voltage range -15 to + 40 mV with the greatest reduction occurring where the amplitude was maximal (ca + 10 mV). In the presence of SOM, the Ca2+ current rising phase was slower and biphasic at potentials between 0 and +40 mV. 3. Application of 0 .cntdot. 1 .mu.M-SOM for greater than 10 s resulted in a desensitization of the response. During a 4 min application of 0 .cntdot. 1 .mu.M-SOM, Ca2+ current amplitude returned to about 90% of control. A second application of 0 .cntdot. 1 .mu.M-SOM produced less block than the initial application. 4. Concentration-response curves for SOM, somatostatin-14(SOM-14) and somatostatin-28 (SOM-28) were fitted to a single-site binding isotherm. The concentrations producing half-maximal block and the maximal attainable blocks of the Ca2+ current for SOM, SOM-14 and SOM-28 were 3 .cntdot. 3, 5 .cntdot. 4 and 35 nM, respectively and 55, 51 and 54%, respectively. SOM-14 ad SOM-28 slowed the CA2+ current rising phase in a manner similar to that of SOM-14 and SOM-28 slowed the CA2+ current rising phase in a manner similar to that of SOM. Somatostatin-28[1-12] had no effect on the CA2+ current at 1 .mu.M. 5. The magnitude of the CA2+ current block produced by 0 .cntdot. 1 .mu.M-SOM was not significantly altered in the presence of 1 .mu.M-idazoxan, atropine, naloxone or the somatostatin antagonist aminoheptanoyl-Phe-D-Trp-Lys-O-benzyl-Thr. 6. Internal dialysis with solutions containing 500 .mu.M-guanylyl-imidodiphosphate (Gpp(NH)p) or guanosine-5''-O-(3-thiotriphosphate)GTP -.gamma.-S) decreased the CA2+ current amplitude by 36 and 41%, respectively, and induced a biphasic rising phase in the CA2+ current. Under these conditions, application of 0 .cntdot. 1 .mu.M-SOM produced significantly less block of Ca2+ current amplitude (7 .cntdot. 1 and 14 .cntdot. 7%, respectively) when compared with controls. 7. Internal dialysis with solutions containing 500 .mu.M-guanosine-5''-O-(2-thiodiphosphate)(GDP-.alpha.-S) had no significant effect on either the CA2+ current amplitude or block produced by 0 .cntdot. 1 .mu.M-SOM. 8. Internal dialysis with solutions containing 500 .mu.M-cyclic adenosine 3'',5''-monophosphate (cyclic AMP) and 3-isobutyl-1-methylxanthine had no significant effect on either the CA2+ current block produced by 0 .cntdot. 1 .mu.M-SOM or the CA2+ current amplitude. 9. Treatment of SCG neurones in short-term culture with 200 ng/ml pertussis toxin (PTX) for 12-16 h greatly reduced the ability of 0 .cntdot. 1 .mu.M-SOM to produce Ca2+ current block. Mean Ca2+ current block was 4 .cntdot. 9 and 40 .cntdot. 8% for PTX-treated and control cultures, respectively. 10. These results suggest that somatostatin blocks a Ca2+ current in adult rat SCG neurones via a pertussis toxin-sensitive guanine nucleotide regulatory protein. This effect appears to be independent of intracellular cyclic AMP.