The Ca2+ antagonists, Mn2+, Co2+, D 600 (α-isopropyl-α-[(N-methyl-N-homoveratryl)-γ-aminopropyl]-3,4,5-trimethoxyphenylacetonitrile), and BAY-1040 (2,6-dimethyl-3,5-dicarbo-methoxy-4-(2-nitro)phenyl-1,4-dihydropyridine) inhibited the response of guinea pig ileal longitudinal muscle to the muscarinic agonist CD (cis-2-methyl-4-dimethylaniinomethyl-1,3-dioxolane methiodide). With the exception of Tm3+ these agents were more effective against the tonic component of response.BAY-1040 was equally effective against phasic components of CD and K+ responses and, although more potent against tonic responses, also failed to discriminate between the tonic components of response to CD and K+. D 600, although less potent than BAY-1040, behaved similarly to BAY-1040 in failing to discriminate between CD and K+ responses. These data suggest that CD and K+ utilize identical, or at least very similar, sources of Ca2+ to sustain excitation-contraction coupling. The tonic component of the CD response showed an apparently cooperative dependence on Ca2+ concentration but D 600 and BAY-1040 behaved as powerful competitive antagonists against Ca2+ in both components of the response to CD (D 600, KI, 1.2 × 10−7 M (phasic), 9.8 × 10−9 M (tonic); BAY-1040, KI,1.8 × 10−8 M (phasic), 4.8 × 10−9 M (tonic)) with potencies similar to those found in other tissues where these agents inhibit membrane Ca2+ currents. The complete additivity of antagonism produced by combination of equieffective concentrations of Tm3+ and D 600 or BAY-1040 indicated that Tm3+ acts at a site distinct from that occupied by D 600 or BAY-1040. In contrast, D 600 and BAY-1040 appear to interact at a common site. No increase in total 45Ca uptake in the presence of 5 × 10−7 M CD could be detected at 0.5 min (phasic component) or 10 min (tonic component) but a significant decrease was found at 5 min and significant increases were found at later times (30 min, 60 min). However, an increase in the 45Ca cellular content, measured after washing in La3+ to remove superficial Ca2+ and to retain cellular Ca2+, could be detected at 0.5 and 10 min in the presence of 5 × 10−7 M CD, in tissues preequilibrated in 46Ca2+ for 60 min. This increase in cellular 46Ca was blocked by atropine, D 600, BAY-1040, and Tm3+ at concentrations that blocked mechanical response. It is concluded that the mechanical responses of guinea pig ileal longitudinal smooth muscle produced by muscarinic agonists or by KCl use Ca2+ that enters through Ca2+ channels similar to those described in other excitable tissues.
Muscarinic agonist potencies at three different effector systems linked to the M2
or M3
receptor in longitudinal smooth muscle of guinea-pig small intestine