EFFECTS OF K+ AND Ca2+ IONS ON MOTILITY AND PHOTOSENSORY RESPONSES OF STENTOR COERULEUS

Extracellular K+ ions above a critical concentration induce ciliary reversal in unstimulated Stentor coeruleus and suppress step‐up photophobic response. This threshold concentration of K+ ions depends on the extracellular Ca2+ concentration, and the subsequent backward gyration and light‐sensitivity suppression seem to depend on the relative concentrations of K+ and Ca2+. The concentration of Ca2+ necessary to overcome K+‐mediated inhibition of phobic response and backward swimming increases non‐linearly with increasing K+ concentration. The Ca2+‐blocking agent. D‐600, selectively inhibits photophobic responses of Stentor, thus further confirming the role of Ca2+ ions in photosensory transduction of this ciliate.