Camp modulation of a ca2+-dependent k+ conductance in rat submandibular acinar cells. Ishikawa, Toru. Department of Molecular Physiology, National Institute for Physiological Sciences, Okazaki, 444 Japan
APStracts 3:0195G, 1996.
Rat submandibular acinar cells exhibit a tetraethylammonium (TEA) -insensitive, Ca2+- activated K+ conductance, which may play an important role in Ca2+-dependent fluid secretion by this gland (Ishikawa et al, 1994; Ishikawa and Murakami, 1995). We have now investigated whether this conductance would be modified during the potentiation of Ca2+-activated fluid secretion by an adenosine 3',5' -cyclic monophosphate (cAMP)-mediated mechanism in the rat submandibular gland. In isolated, vascularly perfused rat submandibular gland, we found that the adenylate cyclase activator forskolin (10 and 30 [mu]M) enhanced a transient initial phase of a biphasic fluid secretion induced by Ca2+-ionophore, A23187 (3 [mu]M). We also unexpectedly found that forskolin reduced a smaller sustained phase of the secretion. The dual effects of forskolin were mimicked by the application of the cocktail of Isobutyl-methyl-xanthine (IBMX) (0.1 mM) and dibutyryl cAMP (dbcAMP) (0.1 mM). Basolateral K+ efflux studies showed that (i) forskolin significantly enhanced a large transient net K+ efflux induced by A23187 in dose-dependent manner, (ii) the forskolin-induced enhancement was also mimicked by a cocktail of IBMX and dbcAMP, and (iii) the A23187-induced K+ efflux enhanced by these agents was not inhibited by TEA (5 or 10 mM), a concentration which is known to completely block Ca2+, voltage -dependent large conductance K+ channels (maxi-K+ channels). Using microspectrofluorometry with fura-2, we then showed that an increase in cytosolic Ca2+ concentration induced by the non-fluorescent, brominated derivative 4-Br-A23187 (3 [mu]M) was not altered by forskolin (30 [mu]M) in freshly isolated rat submandibular acinar cells. Using whole-cell patch-clamp techniques, we demonstrated that a TEA-insensitive K+ conductance induced by A23187 in cells dialyzed with pipette solutions containing cAMP (0.1 mM) was significantly larger than that in cells with control pipette solutions. Taken together, these results indicate that potentiation of the Ca2+ -activated fluid secretion by a cAMP-mediated mechanism is accompanied by that of a TEA-insensitive, Ca2+-activated K+ conductance in this gland. 

Received 31 May 1996; accepted in final form 5 September 1996.
APS Manuscript Number G217-6.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996