SUMMARYRecent studies of the control of stomatal aperture by abscisic acid (ABA) have suggested that its cellular action depends on the availability of calcium ions, which appear to function as secondary messengers. Stimulus-response coupling may, therefore, be similar to that found in many animal tissues.We report here preliminary evidence that ABA has a profound efTect on some mammalian tissues and on a cyanobacterium. Studies on smooth muscles from the vas deferens and bladder of rat showed that 10"" M ABA enhanced field stimulation responses by about 25 %. This effect was inhibited by the calcium-channel blocker, nifedipine. In K"^-depolarized bladder smooth muscle in which the fast calcium channels were voltage-inactivated, 10"" M ABA augmented contracture tension, and enhanced the slow tonic phase of the response which is known to be dependent on the activity of slow calcium channels. Comparative studies using ABA and the synthetic calcium agonist BAY K 8644 suggested that there are close parallels in the mode of action of the two compounds. Heal smooth muscle was remarkably sensitive to ABA, showing enhancement of K+ contracture tension by up to 400% at only 10"" to 10"'* M ABA.In cultures of Nostoc 6720, low concentrations of ABA (about 10"' M) caused a substantial increase in the heterocyst/vegetative cell ratio. Addition of ABA further enhanced the increase in heterocyst content caused by increases in the concentration of Ca^"^ in the medium. These observations provide further support for the proposal that ABA functions as a calcium agonist, a concept consistent with similarities between the action of ABA and the calcium ionophore A23187. In cultures of Nostoc 6720, high (> 10"^ M) concentrations of ABA had an inhibitory effect, and some evidence of a similar dose-response relationship was found in mammalian tissues.Preliminary studies using "•'Ca''+ suggested that ABA stimulates the influx of calcium into both vasa deferentia and Nostoc 6720. It is concluded that ABA may function as a calcium agonist when applied to a wide range of prokaryotic and eukaryotic cells. A recent observation that it is present in the brain of some mammals suggests that this 'plant hormone' may be of much wider biological significance than hitherto recognized. The similar responses of plant, animal and cyanobacterial cells to ABA might indicate the presence of cellular mechanisms which have been genetically conserved through many stages of evolution.

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SUMMARY1. The effects of graded concentrations of caffeine on the Na efflux were investigated. External application of 10 mm caffeine usually caused a biphasic response, viz, a fall, followed by a rise in the Na efflux. 1 and 5 mm caffeine usually caused stimulation. Only the stimulator phase of this response depended on the presence of external Ca2+.2. Internal application of 100 mm~caffeine caused a small rise in the Na efflux, the magnitude of which was independent of external Ca2+ and comparable to that obtained with external application of 1 mm caffeine. This action, however, could be greatly augmented by pre-treating the fibre with x 10-5 M ouabain.3. The rise in Na efflux caused by external application of 10 mmc affeine could be greatly augmented by pre-treating the fibre with 5 x IO--M ouabain. The observed stimulatory response was biphasic, more so in the absence of external Ca2+. Restoration of external Ca2+ following the onset of the second stimulatory phase resulted in further rise of the Na efflux. Measurements of the Na efflux during treatment with graded concentrations of ouabain and 10 mm~caffeine showed that the rate coefficient for Na efflux varied with the ouabain concentration in the range 10-8_10-4 M. Measurements of the ouabain-insensitive Na efflux before and during treatment with 10 mmw caffeine in bathing media containing varying concentrations of Ca, disclosed the existence of two Ca2+_ thresholds, one in the 0-2-5 mm range and the other in the 12-5-15 mm range.4. Comparisons were made between the effects on the Na efflux of * Permanent address: Department of Biological Sciences, University-of Lancaster, Bailrigg, Lancaster LAI 4YQ, England.1-2 2~~E. EDWARD BITTAR AND OTHERS 10 mm caffeine followed by external acidification, and external acidification, followed by 10 mm caffeine. The magnitude of the response of the ouabaininsensitive Na efflux to external acidification before treatment with 10 mm caffeine was greater than that found when external acidification followed external application of the alkaloid. It also was considerably greater than that of the response to external application of 10 mm caffeine before external acidification.5. External application of 10 mm procaine prevented 10 mmw caffeine from stimulating the Na efflux, and from inducing contractures. Internal application of 100 mm-EGTA reduced the response of the Na efflux to 10 mmw caffeine, and also prevented the fibre from contracting. External application of 10-4m diphenylhydantoin reduced the response of the Na efflux to 10 mmw caffeine but failed to prevent the development of contractures.6. Internal application of 0-05 m-cGMP, cAMP or its dibutyryl derivative caused a large rise in the Na efflux. The magnitude of the effects observed in ouabain-poisoned fibres was often greater than that in unpoisoned fibres. Internal application of 2-5 units/ml. phosphodiesterase beforehand failed to reduce the magnitude of the stimulatory response to injected cyclic nucleotides. Injected phosphodiesterase also failed to reduce th...