Contribution of vascular smooth muscle to contractile responses of guinea-pig isolated lung parenchymal strips☆

Abstract Parenchymal strips, isolated from the guinea-pig lung, were stimulated transmurally at different electrical frequencies in vitro. Frequency-response analyses for contraction were obtained on paired strips in the absence or presence of antagonists. Tetrodotoxin was shown to block the effects of electrical stimulation, demonstrating that the contractile response was due to activation of efferent neurons in the tissue. Phentolamine, 10 −5 M, and atropine, 10 −6 M, antagonists demonstrated to be selective for blocking α-adrenergic and muscarinic receptors, respectively, produced inhibition of the contractile effects of electrical stimulation. When analyzed at the level of the frequency required to produce a contraction equal to 10% of the maximum produced by histamine, the EF 10 , the frequency-response curves were shifted 3- to-fold to the right by each receptor antagonist. Combination of the two receptor antagonists produced a greater degree of blockade than either alone and, at the maximum frequency utilized (32 Hz), the contractile response did not reach the EF 10 in all tissues. Propranolol, 10 −6 M, did not alter the effects of electrical stimulation, but increased the magnitude of contraction produced by exogenously applied norepinephrine. In tissues taken from reserpine-pretreated animals, phentolamine did not produce a statistically significant change in the EF 10 for electrical stimulation. The data provide evidence that both vascular and airway smooth muscles are present and contribute to contractile responses in the guinea-pig lung parenchymal strip.