Evidence for a dihydropyridine-sensitive and conotoxin-insensitive release of noradrenaline and uptake of calcium in adrenal chromaffin cells

It has been suggested that neuronal voltage-sensitive calcium channels (VSCC) may be divided into dihydropyridine (DHP)-sensitive (L) and DHP-insensitive (N and T), and that both the L and the N type channels are attenuated by the peptide blocker ω-conotoxin. Here the effects of ω-conotoxin on release of noradrenaline and uptake of calcium in bovine adrenal chromaffin cells were investigated. Release of noradrenaline in response to 25 mM K⁺, 65 mM K⁺, 10 nM bradykinin or 10 μM prostaglandin E₁ was not affected by ω-conotoxin in the range 10 nM-1 μM. ⁴⁵Ca²⁺ uptake stimulated by high K⁺ and prostaglandin was attenuated by 1 μM nitrendipine and enhanced by 1 μM Bay K 8644; these calcium fluxes were not modified by 20 nM ω-conotoxin. With superfused rat brain striatal slices in the same medium as the above cell studies, release of dopamine in response to 25 mM K⁺ was attenuated by 20 nM ω-conotoxin. These results show that in these neurone-like cells, release may be effected by calcium influx through DHP-sensitive but ω-conotoxin-insensitive VSCC, a result inconsistent with the suggestion the ω-conotoxin blocks both L-type and N-type neuronal calcium channels.