Influence of acetylcholine agonists and antagonists on the swelling of etiolated wheat (Triticum aestivum L.) mesophyll protoplasts.

Etiolated wheat (Triticum aestivum L.) mesophyll protoplasts swell within 30 min in darkness after a red light (R) pulse or addition of acetylcholine (ACh), if 0.5 mM CaCl2 is present in the medium. In addition, ACh is also able to induce swelling in the presence of both 0.1 mM KCl or NaCl. Besides ACh, only carbamylcholine out of the choline derivatives tested was active in induction of swelling in the presence of K(+) or Na(+). The K(+)/Na(+)-dependent ACh-induced protoplast swelling was nullified by a 'calmodulin inhibitor', but not by Ca(2+)-channel blockers, Li(+) or VO 4 (3-) . The antagonists atropine (of muscarine-sensitive ACh receptors, mAChRs) andD-tubocurarine (of nicotine-sensitive ACh receptors, nAChRs) nullified the Ca(2+) - and the K(+)/Na(+)-dependent protoplast swelling responses, respectively, while having no effect on the Ca(2+)-dependent R-induced swelling response. Moreover, muscarine and nicotine mimicked ACh in the Ca(2+)- and K(+)/Na(+)-dependent swelling responses respectively. Just as is the case in animal cells, the proposed mAChRs appear to be associated with a phosphatidylinositol-dependent pathway, whereas the proposed nAChRs are phosphatidylinositol independent. Similarity between the action of ACh via the proposed mChRs and R via phytochrome in protoplast swelling indicates they share in common signal-transduction pathway.