EFFECT OF CYCLOPIAZONIC ACID ON CONTRACTIONS PRODUCED BY TACHYKININ NK1 AND NK2 RECEPTOR AGONISTS IN THE CIRCULAR MUSCLE OF GUINEA-PIG COLON

1 This study aimed to assess the effect of cyclopiazonic acid (CPA), a n inhibitor of sarcoplasmic reticulum calcium (Ca) pump, against contr actile responses produced by selective tachykinin NK1 and NK2 receptor agonists, [Sar(9)]substance P (SP) sulfone and [beta Ala(8)]neurokini n A (NKA) (4-10), respectively, on the circular muscle of guinea-pig c olon. All experiments were performed in the presence of atropine (1 mu M) and indomethacin (10 mu M). 2 In organ bath experiments, a submaxi mally equieffective concentration of the two agonists (10 nM) was sele cted: [Sar(9)]SP sulfone (10 nM) produced a biphasic contraction, the two amplitudes averaging 75 +/- 2 and 43 +/- 3% of the maximal respons e to KCl (80 mM) at 1 and 15 min from application of the agonist, resp ectively. CPA (3 mu M for 60 min) slightly reduced the phasic response to [Sar(9)]SP sulfone (16 +/- 4% inhibition) and markedly suppressed the tonic component (89 +/- 3% inhibition). 3 The contraction produced by [beta Ala(8)]NKA (4-10) (10 nhl) was more sustained than that indu ced by the NK1 receptor agonist: it averaged 69 +/- 5 and 73 +/- 4% of the response to KCl at 1 and 15 min from application of the agonist, respectively. CPA slightly and evenly depressed the response to [beta Ala(8)]NKA (4-10) (18 +/- 7 and 21 +/- 5% inhibition at 1 and 15 min). 4 In the presence of tachykinin NK1 and NK2 receptor antagonists (SR 140333 and MEN 10627, respectively, 1 mu M each) and of L-nitroarginin e (100 mu M), KCl (40 mM) produced a distinct phasic and tonic contrac tion which was suppressed by 1 mM nifedipine. CPA (3 mu M) did not aff ect the phasic contraction to KCl but abolished the tonic component of the response. 5 In the presence of 1 mu M nifedipine, the response to [beta Ala(8)]NKA (4-10) was slightly depressed (32 +/- 6% inhibition) in its early component only, while the response to [Sar(9)]SP sulfone was abolished. CPA produced a slight inhibition (15 +/- 9 and 33 +/- 10% at 1 and 15 min, respectively) of the nifedipine-resistant respons e to [beta Ala(8)]NKA (4-10), an effect similar to that observed in th e absence of nifedipine. Therefore, a large part of the response to [b eta Ala(8)]NKA (4-10) persisted in the presence of both CPA and nifedi pine. 6 In the sucrose gap, a prolonged superfusion with [Sar(9)]SP su lfone (0.1 mu M for 5 min) produced sustained depolarization with supe rimposed spikes and contraction. CPA (3 mu M) produced transient depol arization and contraction. In the presence of CPA, the initial respons es (depolarization, spikes and contraction) to [Sars]SP sulfone were u naffected but the sustained component of contraction was absent; the l atter effect was accompanied by a suppression of spikes while the sust ained depolarization was present. 7 We conclude that, during sustained depolarization produced by the NK1 receptor agonist, blockade of the sarcoplasmic reticulum Ca pump by CPA produces a faster Ca-dependent i nactivation of Ca channels, thereby eliminating spikes and abolishing the tonic component of contraction. Ca mobilization/reuptake from a CP A-sensitive store seems to be of minor importance for regulating the N K2 receptor-mediated contractile responses.