Prostaglandin modulation of Ca2+ channels in rat sympathetic neurones is mediated by guanine nucleotide binding proteins.

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Abstract
1. The effects of prostaglandins on whole-cell Ca2+ currents of acutely isolated and short-term cultured adult rat superior cervical ganglion neurones were investigated using the patch-clamp technique. 2. Prostaglandin E2 (PGE2) produced a rapid, reversible and concentration-dependent reduction of the sympathetic neurone Ca2+ current. The effects of PGE2 were both voltage and time dependent. The relationship between Ca2+ current inhibition and test potential was 'bell' shaped with maximal inhibition occurring near the potential where the Ca2+ current amplitude was maximal (ca + 10 mV). In the presence of PGE2, the Ca2+ current rising phase was slower and biphasic at potentials between 0 and +40 mV. 3. Prolonged (> 2 min) application of 1 microM PGE2 resulted in a desensitization of the response. Similarly, repeated short (ca 1 min) applications of 1 microM PGE2 resulted in a progressive tachyphylaxis of the response. 4. A concentration-response curve for PGE2 was well described by a single-site binding isotherm. The concentration producing half-maximal block (IC50) and the maximal attainable reduction of the Ca2+ current were 7.8 nM and 48%, respectively. 5. When compared at a concentration of 1 microM, PGF2 alpha was less potent (33% inhibition) than PGE2 but otherwise produced similar effects. In contrast, 1 microM PGD2 had negligible effects. 6. Activation curves, as derived from tail current amplitudes, were described by the sum of two Boltzmann functions in both the presence and absence of PGE2. In the presence of PGE2, the activation curve was shifted toward more depolarized potentials. Most of the shift could be accounted for by a decrease in the fractional amplitude of the current component activated at hyperpolarized potentials along with a concomitant increase in the component activated at depolarized potentials. The deactivation time constant (0.33 ms), measured at -40 mV, was not altered by PGE2. 7. The majority of the Ca2+ current inhibition produced by PGE2 was relieved by depolarizing conditioning pre-pulses to +80 mV for 50 ms. 8. Dialysis of sympathetic neurones with a pipette solution containing 2.0 mM guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) abolished the effects of PGE2 on the Ca2+ current. Pretreatment of the neurones overnight with pertussis toxin significantly, but incompletely, decreased the Ca2+ current inhibition produced by PGE2. 9. The prolonged Ca2+ tail current component induced by the dihydropyridine Ca2+ channel 'agonist' (+)202-791 (2 microM) was unaffected by 1 microM PGE2. 10. PGE2 partially inhibited the Ca2+ current component remaining after pretreatment of the neurones with 10 microM omega-conotoxin.(ABSTRACT TRUNCATED AT 400 WORDS)