A new class of inhibitors of cAMP-mediated Cl- secretion in rabbit colon, acting by the reduction of cAMP-activated K+ conductance.
Previously we have shown that arylaminobenzoates like 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), which are very potent inhibitors of NaCl absorption in the thick ascending limb of the loop of Henle, are only poor inhibitors of the cAMP-mediated secretion of NaCl in rat colon. This has prompted our search for more potent inhibitors of NaCl secretion in the latter system. The chromanole compound 293 B inhibited the equivalent short-circuit current (Isc) induced by prostaglandin E2 (n = 7), vasoactive intestinal polypeptide (VIP, n = 5), adenosine (n = 3), cholera toxin (n = 4) and cAMP (n = 6), but not by ionomycin (n = 5) in distal rabbit colon half maximally (IC50) at 2 mumol/l from the mucosal and at 0.7 mumol/l from the serosal side. The inhibition was reversible and paralleled by a significant increase in transepithelial membrane resistance [e.g. in the VIP series from 116 +/- 16 omega.cm2 to 136 +/- 21 omega.cm2 (n = 5)]. A total of 25 derivatives of 293 B were examined and structure activity relations were obtained. It was shown that the racemate 293 B was the most potent compound within this group and that its effect was due to the enantiomer 434 B which acted half maximally at 0.25 mumol/l. Further studies in isolated in vitro perfused colonic crypts revealed that 10 mumol/l 293 B had no effect on the membrane voltage across the basolateral membrane (Vbl) in non-stimulated crypt cells: -69 +/- 3 mV versus -67 +/- 3 mV (n = 10), whilst in the same cells 1 mmol/l Ba2+ depolarised Vbl significantly. However, 293 B depolarised Vbl significantly in the presence of 1 mumol/l forskolin: -45 +/- 4 mV versus -39 +/- 5 mV (n = 7). Similar results were obtained with 0.1 mmol/l adenosine. 293 B depolarised Vbl from -40 +/- 5 mV to -30 +/- 4 mV (n = 19). This was paralleled by an increase in the fractional resistance of the basolateral membrane. VIP had a comparable effect. The hyperpolarisation induced by 0.1 mmol ATP was not influenced by 10 mumol/l 293 B: -75 +/- 6 mV versus -75 +/- 6 mV (n = 6). Also 293 B had no effect on basal K+ conductance (n = 4). Hence, we conclude that 293 B inhibits the K+ conductance induced by cAMP. This conductance is apparently relevant for Cl- secretion and the basal K+ conductance is insufficient to support secretion.
Lohrmann
E
E
Physiologisches Institut der Albert-Ludwigs-Universität, Freiburg, Germany.
Burhoff
I
I
Nitschke
R B
RB
Lang
H J
HJ
Mania
D
D
Englert
H C
HC
Hropot
M
M
Warth
R
R
Rohm
W
W
Bleich
M
M
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D018118:Chloride Channels / Q000037:antagonists & inhibitors*; D002712:Chlorides / Q000378:metabolism*; D003106:Colon / Q000187:drug effects*; D000242:Cyclic AMP / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005260:Female; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000187:drug effects*; D011817:Rabbits; D014660:Vasoactive Intestinal Peptide / Q000494:pharmacology
D018118:Chloride Channels; D002712:Chlorides; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D014660:Vasoactive Intestinal Peptide; D000584:Amiloride; D000242:Cyclic AMP