Contractions mediated by α1‐adrenoceptors and P2‐purinoceptors in a cat colon circular muscle
Authors: Venkova Kalina , Milne Andrew , Krier Jacob 
Published: July 2012

The postjunctional excitatory and inhibitory effects of adrenoceptor and purinoceptor agonists and antagonists were studied in circular smooth muscle strips of cat colon.
In the presence of tetrodotoxin (0.5 μm), noradrenaline caused contraction or relaxation of circular smooth muscle at resting tension or with raised tone, respectively. The noradrenaline‐evoked contractions were potentiated and the noradrenaline‐evoked relaxations were antagonized by propranolol (1 μm), suggesting β‐adrenoceptor involvement.
At resting tension, noradrenaline, adrenaline and the selective α1‐adrenoceptor agonist, phenylephrine, caused concentration‐dependent contractile responses, with EC50 values of 1.8 ± 0.2 μm, 1.9 ± 0.4 μm and 4.3 ± 1.7 μm, respectively. The EC50 values and the amplitude of maximal responses were not significantly different from one another. Clonidine (0.1–500 μm), a selective α2‐adrenoceptor agonist, was not effective.
Prazosin (0.1–9 μm), competitively antagonized the contractile effects of noradrenaline with an estimated pA2 value of 6.93 and a slope of 1.07 ± 0.03. The Kb values, estimated from a single shift (0.1 μm prazosin) of the concentration‐response curves to noradrenaline, adrenaline and phenylephrine were 92.8 ± 9.3 nM, 108.7 ± 6.4 nM and 18.4 ± 3.1 nM, respectively.
At resting tension, adenosine 5′ triphosphate (ATP, 5–1000 μm), α,β‐methylene adenosine 5′‐triphosphate (α,β‐MeATP, 0.05–50 μm), β,γ‐methylene adenosine 5′‐triphosphate (β,γ‐MeATP, 0.5–100 μm), and 2‐methylthioadenosine 5′‐triphosphate (2‐MeSATP, 1–500 μm) caused concentration‐dependent contractions with EC50 values of 60.5 ± 15.9 μm, 0.7 ± 0.1 μm, 7.6 ± 0.1 μm and 25.3 ± 12.8 μm, respectively. The maximal responses to α,β‐MeATP and β,γ‐MeATP were greater than maximal responses to 2‐MeSATP and ATP.
Suramin (50–500 μm), competitively antagonized the contractile responses of α,β‐MeATP with an estimated pA2 value of 4.92 and a slope of 1.08 ± 0.04. The Kb values, estimated from a single shift (100 μm suramin) of the concentration‐response curves to ATP, α,β‐MeATP, β,γ‐MeATP and 2MeSATP were 52.3 ± 20.2 μm, 25.2 ± 4.5 μm, 21.7 ± 11.0 μm and 11.6 ± 2.7 μm, respectively.
At resting tension, reactive blue 2 (100 μm), a selective antagonist of the P2Y‐purinoceptor, and 8‐(p‐sulphophenyl)‐theophylline (8‐SPT) (1 μm), a selective antagonist of the P1‐purinoceptor, did not antagonize the contractile responses to α,β‐MeATP (0.5–5 μm). Contractile responses to ATP (50–500 μm) were not altered by 8‐SPT (1 μm) but were potentiated by reactive blue 2 (100 μm).
With raised tone, ATP and 2‐MeSATP caused a relaxant effect. This effect of ATP was not altered by either tetrodotoxin (TTX) (0.5 μm) or suramin (100 μm), but was antagonized by reactive blue 2 (100 μm) and 8‐SPT (1 μm), suggesting that inhibitory P2Y‐ and P1‐purinoceptors are involved. In contrast, α,β‐MeATP and β,γ‐MeATP caused only contractions. This contractile effect of α,β‐MeATP was resistant to TTX (0.5 μm) and antagonized by suramin (100 μm).
In summary, cat colon circular muscle contains postjunctional α1‐adrenoceptors and P2X‐purinoceptors which mediate contractions and P2Y‐ and P1‐purinoceptors which mediate relaxation. Postjunctional α2‐adrenoceptors appear not to be present.