Vicia faba epidermal peels treated with adenosine-3', 5'-cyclic monophosphorothioate, Sp isomer (Sp-cAMPS), a cyclic AMP agonist and activator of cyclic AMP-dependent protein kinases, showed a relatively rapid and marked enhancement of stomatal aperture in light as well as in darkness, with a saturating response in the micromolar range. This concentration produced a detectable decrease in cytosolic calcium concentration ([Ca2+](c)) in guard cell protoplasts within a very short time (1-3 min.), whereas with 1 nM Sp-cAMPS no variation in [Ca2+](c) was detected during the same period nor was any variation observed in [Ca2+](c) in guard cell protoplasts incubated with 1 mu M adenosine-3', 5'-cyclic monophosphorothioate, Rp-isomer (Rp-cAMPS), a competitive inhibitor of the protein kinase A. Rp-cAMPS not only inhibited the kinetic riboside-induced stimulation of stomata opening but also produced a marked inhibition of light-induced stomatal opening. Rp-cAMPS (10 mu M) was effective in antagonizing the effects of Sp-cAMPS on stomatal opening and on the level of cytosolic calcium. The present results constitute evidence that these nucleotides influence the endogenous activity of protein kinases in the underlying mechanism of stomatal movement and are related to a hypothetical light- and/or hormone-signal-transduction pathway in these cells, which in turn could be related to the cytosolic calcium status.
