Effects of butoxybenzyl hyoscyamine bromide (BBHB) on contractile response of isolated guinea-pig gallbladder to specific and non-specific stimulants were examined. Results are as follows:
1) BBHB, in low concentrations (10
-9_??_10
-6M), exerted an atropine-like competitive blocking action against the acetylcholine (ACh)-induced contraction. The affinity to muscarinic receptor of BBHB (
pA2=8.79±0.08, molar unit) was greater than that of atropine (
pA2=7.64±0.12, molar unit).
2) BBHB, in high concentrations (5×10
-6_??_4×10
-4M), showed non-competivity blocking action against the KCl-induced contraction (
pD2'=3.30±0.06) and the BaCl
2-induced contraction (
pD2'=3.12±0.15). The affinity of BBHB was somewhat less than that of papaverine (
pD2'=4.80±0.11 to BaCl
2 and 4.81±0.05 to KCl).
3) In the Ca
++-free medium, high concentrations of BBHB inhibited noncompetitively the BaCl
2-induced contraction, the
pD2' value being 3.05±0.12, however BBHB, even in a high concentration, did not affect the Ca
++-induced contraction of K
+-depolarized preparation in Ca
++-free medium. On the other hand, papaverine antagonized noncompetitively the Ba
++-induced contraction (
pD2'=4.56±0.21) and the Ca
++-induced contraction (
pD2' =4.75 ±0.03).
These results suggest that BBHB, in low concentrations, antagonizes competitively the ACh-induced contraction at muscarinic receptors, whereas in gigh concentrations reduced K
+-induced and Ba
++-induced contractions, presumably by interfering with the K
+- and Ba
++-induced inward release of membrane-bound calcium, by decreasing the Ba
++-influx and/or by preventing the barium ion from being utilized by the contractile proteins.
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