SUBSTANCE-P POTENTIATES CALCIUM-CHANNEL MODULATION BY SOMATOSTATIN INCHICK SYMPATHETIC-GANGLIA

1. The whole cell patch clamp was used to measure calcium current in i solated chick sympathetic ganglion neurons. Previous results showed th at somatostatin inhibits calcium currents (I-Ca) in a voltage-dependen t manner. The effect of somatostatin rapidly desensitizes. In addition , the action of somatostatin on the calcium current is inhibited by ac tivation of protein kinase C (PKC). Because substance P (SP) has been shown to activate PKC in the chick sympathetic neurons, we here test t he effects of SP on the calcium current and on the modulatory action o f somatostatin. 2. At a concentration of 1 mu M, SP had small, variabl e effects on I-Ca. 3. SP in the presence of guanosine 5'-triphosphate- gamma-S, or at higher concentrations (10 mu M), inhibited I-Ca in a vo ltage-dependent manner, similar to the action of somatostatin. 4. Rath er than inhibiting the action of somatostatin, SP (1 mu M) potentiated the response to somatostatin. This effect of SP was only observed aft er the response to somatostatin had partially desensitized. SP had no effect on nondesensitized responses to somatostatin. 5. Desensitizatio n of the somatostatin response involved a shift in its dose-response c urve toward higher somatostatin concentrations as well as a decrease i n the maximal response. SP appears to counteract the shift of the dose -response curve selectively. 6. The potentiation of the somatostatin r esponse by SP is blocked by 1-(5-isoquinolinylsulfonyl)-2-methylpiperz ine (H-7), but not by Calphostin C, Compound 5, k252a, protein kinase C (PKC)(19-36), or adenylyl-imidodiphosphate (AMP-PNP), suggesting tha t phosphorylation is not involved and that the H-7 action does not dep end on kinase inhibition. 7. The action of SP does not appear to invol ve dephosphorylation either, because its action was not blocked by the phosphatase inhibitors calyculin A or okadaic acid. 8. These results suggest a novel mechanism for interactions between two neurotransmitte rs in which one transmitter potentiates the action of a second by coun teracting the effects of desensitization.