Distinctive pharmacological profile of a nonadrenergic inhibitory system in bullfrog lung

Epinephrine stimulates prostaglandin synthesis by bullfrog lung from warm-acclimated, but not cold-acclimated, animals

Exogenous prostaglandins (PGs) have been shown to have differing effects on frog lung contractility. In this study, prostaglandin synthesis was measured in lung tissues from warm-acclimated (WA, 22 degrees C) and cold-acclimated (CA, 5 degrees C) American bullfrogs, Rana catesbeiana, incubated for 30 min at 5 degrees or 22 degrees C. Media were assayed by radioimmunoassay for PGE2, PGF2 alpha, 6-keto PGF 1 alpha (the metabolite of PGI2), and thromboxane (TX)B2 (the metabolite of TXA2). PGE2 was produced in greatest quantity by tissues from WA and CA animals, at both incubation temperatures. Epinephrine stimulated PGE2, PGF2 alpha, and TXB2 synthesis at 22 degrees C but only stimulated PGE2 production at 5 degrees C. In tissues from CA frogs, epinephrine did not stimulate prostaglandin synthesis at either incubation temperature. Ibuprofen (10(-5) M) inhibited basal and epinephrine-stimulated prostaglandin synthesis in tissues from WA frogs incubated at 22 degrees C. The beta receptor antagonist propranolol (10(-6) M) blocked the epinephrine-stimulated synthesis of PGE2, PGF2 alpha and TXB2, suggesting epinephrine stimulates prostaglandin synthesis through beta receptor activation. The absence of stimulation by epinephrine in lung from CA animals, but not in 5 degrees C incubations of tissues from WA animals, suggests that a modification of beta receptors occurs during prolonged cold exposure.

Effect of quinidine and tetrodotoxin on the activation of non-adrenergic nerves in guinea-pig trachealis muscle

Activation of non-adrenergic neurones in guinea-pig trachealis muscle was accomplished by electrical field stimulation and by the neurotoxin aconitine, in the presence of atropine and propranolol. Aconitine (10(-5) M) activated non-adrenergic neurones more slowly, but was as efficacious as supramaximal field stimulation (70 V, 1 msec, 1-100 Hz), producing 70-80% of the maximal relaxation to forskolin or theophylline. Quinidine (3 X 10(-5) M-3 X 10(-4) M) and tetrodotoxin (5 X 10(-9) M-3 X 10(-6) M) blocked relaxations to aconitine and field stimulation, without affecting smooth muscle relaxant responses to forskolin. The results suggest that the non-adrenergic inhibitory effects of quinidine are related to its presynaptic local anaesthetic actions, rather than to postsynaptic receptor blockade of the non-adrenergic inhibitory neurotransmitter.

Mechanisms of intrinsic tone in bullfrog lung: relaxant effects of indomethacin, ouabain and potassium

Bullfrog hemilungs showed minimal relaxation (9 +/- 2% of the maximal relaxant effect of theophylline, Imax) after a 16 h incubation in 10(-5) M indomethacin, indicating that prostaglandin synthesis plays little or no role in the high intrinsic tone characteristic of this preparation. A higher concentration of indomethacin (10(-4) M) produced greater relaxation (23 +/- 3% of Imax), but also markedly potentiated isoprenaline-induced relaxation. The interaction with isoprenaline was similar to that previously found for papaverine, a phosphodiesterase inhibitor. Ouabain (10(-5) and 10(-4) M) produced an initial contraction followed by marked relaxation (50% of Imax), indicating that a ouabain-sensitive mechanism is of major importance in the maintenance of intrinsic tone. Ouabain-treated hemilungs showed reversal (relaxation) of the normal contractile response to 26 mM potassium and marked impairment of the contractile response to calcium in calcium-depleted preparations. These effects suggest that ouabain-induced relaxation reflects a drug action on calcium movements. The marked relaxation (30 to 40% of Imax) produced by 26 mM potassium in ouabain-treated hemilungs is of particular interest in that it indicates a mechanism of potassium-induced relaxation distinct from stimulation of sodium-potassium ATPase.

Effect of stimulation of non-adrenergic inhibitory nerves on cyclic nucleotide levels in bullfrog lung

Carbachol in the presence of atropine and propranolol was employed to stimulate a non-adrenergic neural inhibitory system in the hemilung of the bullfrog (Rana catesbeiana). Tissue levels of cGMP were elevated 95% by carbachol whilst cAMP levels were unchanged. The phosphodiesterase inhibitor papaverine did not affect either cAMP or cGMP levels, but did selectively increase the carbachol-induced increase in lung cGMP to 220% of control levels. Papaverine did not potentiate the relaxant effects of carbachol. The results suggest that cyclic nucleotides may not be directly involved in the relaxation produced by stimulation of the non-adrenergic neural inhibitory system in this preparation.