The effects of Ca²⁺ antagonists on mechanical responses and Ca²⁺ movements in guinea pig ileal longitudinal smooth muscle

The Ca 2+ antagonists, Mn 2+ , Co 2+ , 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 Tm 3+ 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 Ca 2+ to sustain excitation-contraction coupling. The tonic component of the CD response showed an apparently cooperative dependence on Ca 2+ concentration but D 600 and BAY-1040 behaved as powerful competitive antagonists against Ca 2+ in both components of the response to CD (D 600, K I , 1.2 × 10 −7 M (phasic), 9.8 × 10 −9 M (tonic); BAY-1040, K I ,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 Ca 2+ currents. The complete additivity of antagonism produced by combination of equieffective concentrations of Tm 3+ and D 600 or BAY-1040 indicated that Tm 3+ 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 45 Ca 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 45 Ca cellular content, measured after washing in La 3+ to remove superficial Ca 2+ and to retain cellular Ca 2+ , could be detected at 0.5 and 10 min in the presence of 5 × 10 −7 M CD, in tissues preequilibrated in 46 Ca 2+ for 60 min. This increase in cellular 46 Ca was blocked by atropine, D 600, BAY-1040, and Tm 3+ 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 Ca 2+ that enters through Ca 2+ channels similar to those described in other excitable tissues.