Different Ca 2+ influx pathways mediate tachykinin receptor-induced contraction in circular muscle of guinea-pig colon

Abstract

We used an electrophysiological approach (single sucrose gap) to compare the mechanism of action of selective tachykinin NK 1 and NK 2 receptor agonists ([Sar 9]substance P sulfone and [βAla 8]neurokinin A-(4–10), respectively) in producing contraction of the circular muscle of the guinea-pig proximal colon. [Sar 9]Substance P sulfone produced a marked depolarization, action potentials and increase in membrane conductance. On the other hand, [βAla 8]neurokinin A-(4–10) produced less depolarization of the cell membrane and did not change membrane resistance. Nifedipine (1 μM) greatly reduced (80% inhibition) the contraction due to [Sar 9]substance P sulfone while that due to [βAla 8]neurokinin A-(4–10) was slightly affected (13% inhibition). Action potentials induced by either agonist were suppressed by nifedipine, while depolarization was reduced only to a minor extent. When tested in a Ca 2+-free medium, the contraction produced by either agonist was greatly reduced (84–89%) as compared to the control. In organ bath experiments [Sar 9]substance P sulfone and [βAla 8]neurokinin A-(4–10) produced concentration-dependent contraction of the circular muscle of the colon (EC 50 8 and 12 nM, respectively). Nifedipine (1 μM) markedly suppressed the response to [Sar 9]substance P sulfone while that to [βAla 8]neurokinin A-(4–10) was only slightly depressed. These findings demonstrate that NK 1 receptor-mediated contraction is strictly linked to membrane depolarization and action potentials generation through nifedipine-sensitive Ca 2+ channels (electromechanical coupling) while the NK 2 receptor-mediated contraction is substantially unrelated to depolarization and, while being largely dependent upon extracellular Ca 2+, is nifedipine-resistant, possibly linked to the opening of non-selective (Ca 2+-permeable) receptor-gated cation channels (pharmacomechanical coupling).