EFFECTS OF CA2+ ANTAGONISTS ON MECHANICAL RESPONSES AND CA2+ MOVEMENTS IN GUINEA-PIG ILEAL LONGITUDINAL SMOOTH-MUSCLE

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-dicarbomethoxy-4-(2nitro)phenyl-1,4-dihydropyridine) inhibited the response of guinea pig ileal longitudinal muscle to the muscarinic agonist CD (cis-2-methyl-4-dimethylaminomethyl-2,3-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 powerful competitive antagonists against Ca2+ in both components of the response to CD (D 600, K(I), 1.2x10-7 M (phasic), 9.8x10-9 M (tonic); BAY-1040, K(I), 1.8x10-8 M (phasic), 4.8x10-9 M (tonic) with potencies to those found in 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 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 45Ca uptake in the presence of 5x10-7 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 5x10-7 M CD, in tissues preequilibrated in 45Ca2+ for 60 min. This increase in cellular 45Ca 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.