THE EFFECT OF WATER-SOLUBLE CYANOTOXIN(S) PRODUCED BY 2 SPECIES OF ANABAENA ON THE RELEASE OF ACETYLCHOLINE FROM THE PERIPHERAL CHOLINERGICNERVOUS-SYSTEM OF THE RAT AIRWAY

A water extract of the lyophilised fresh-water alga Anabaena flos-aqua e enhanced substantially the release of [H-3]acetylcholine ([H-3]acety lcholine and [H-3]choline) from cholinergic nerves of rat bronchi. Par allel experiments performed with the related species Anabaena lemmerma nnii did not demonstrate this effect. The effect on the release of [H- 3]acetylcholine by A. flos-aquae extract was concentration dependent. The A. flos-aquae induced [H-3]acetylcholine release was not reduced b y exposure to a low concentration of Ca2+, but omega-conotoxin GVIA (1 .0 mu M), a blocker of N-type Ca2+ channels reduced the release of [3H ]acetylcholine induced by the A. flos-aquae extract. Addition of verap amil in a concentration (1.0 mu M) specific for inhibition of L-type C a2+ channels had no effect on the neurotransmitter release. A reductio n in the release was, moreover, observed with the intracellular Ca2+ c helator BAPTA/AM (30 mu M) and with the Na+ channel blocker tetrodotox in (3.0 mu M). During patch-clamp studies of GH(4)C(1) neuronal cells, which have L- and T-type Ca2+ channels, but no Na+ channels, it was s hown that a water extract of A. flos-aquae depolarised these cells and reduced, rather than enhanced, the influx of Ca2+. Such an effect was not seen following exposure of GH(4)C(1) cells to water extracts of A . lemmermannii. In addition to its presynaptic activity, the water ext ract of A. flos-aquae showed an antimuscarinic effect by displacing [H -3]QNB binding from muscarinic receptors in homogenates of rat bronchi . A similar but more potent effect was observed during experiments wit h water extract of A. lemmermannii. None of the respective water extra cts showed any effects on cholinesterase activities in rat bronchial s mooth muscle. The present observations suggest, therefore, that water extracts of A. flos-aquae may depolarise cells by activation of mono a nd divalent cation channels in cholinergic nerve cells. These channels are probably Na+ channels and N-type, but not L-or T-type Ca2+ channe ls. L-and T-type Ca2+ channels were blocked in experiments with GH(4)C (1) cells and high concentrations of Ca2+ channel blockers were necess ary to reduce the effects of A. flos-aquae extract in cholinergic nerv es in the airways. Furthermore, A. flos-aquae extract may also mobilis e Ca2+ from intracellular compartments. A. lemmermannii, on the other hand, does not contain components which alter mono and divalent cation -fluxes across cell membranes, but may rather have substances with mor e potent antagonistic effects on muscarinic cholinergic receptors than what is observed in experiments with A. flos-aquae. (C) 1998 Elsevier Science B.V.