Effects of low temperature and pharmacological interventions on the responses of the isolated guinea-pig trachea

Summary Cooling the guinea-pig isolated trachea from 37°C to 20°C virtually abolished the response to CaCl₂ (in K⁺-depolarized tissues) and depressed that to histamine (about 75% reduction), KCl and 5-hydroxytryptamine (around 50% inhibition) while the response to acetylcholine remained unaffected. A further cooling to 10°C was necessary to inhibit acetylcholine-induced contractions. Hyporesponsiveness to spasmogens by cooling was not associated with subsensitivity (increased EC₅₀) except for 5-hydroxytryptamine. Contractile responses to KCl (50 mmol/l), histamine (1 mmol/l) and 5-hydroxytryptamine (0.1 mmol/l) in a Ca²⁺-free EGTA (0.1 mmol/l)-containing solution were inhibited by cooling to 20°C but responses to acetylcholine (1 mmol/l) in the same experimental conditions were not affected. Cooling to 20°C after treatment with an antagonist (ouabain 10 µmol/l, amiloride 0.1 mmol/l or vanadate 0.1 mmol/l) or after incubation in K⁺-free medium or low Na⁺ (25 mmol/l) solution produced the same or greater inhibitions of tracheal responses to spasmogens than cooling alone. The guinea-pig trachea treated with phorbol 12,13-diacetate (PDA; 1 µmol/l) and cooled to 20°C responded to spasmogens similarly to a trachea untreated with PDA at 37°C. In contrast, PDA (1 µmol/l) did not counteract the depressed responsiveness to histamine of ouabain (10 µmol/l)- or amiloride (0.1 mmol/l)- treated tracheal strips at 37°C. PDA (1 mol/l) enhanced tracheal contractions caused by KCl (50 mmol/l) in Ca²⁺-free medium at 20°C but failed to augment those to histamine in the same conditions. PDA (0.1 or 1 µmol/l) did not change the concentration-response curve for Ca²⁺ in skinned trachea. In conclusion, low temperature inhibits the responses to spasmogens in the guinea-pig trachea. Probably, reduced temperature interferes with extracellular and intracellular sources of Ca²⁺ which are differently affected by various spasmogens. Alteration by cooling of the electrogenic Na⁺ pump, Na⁺/Ca²⁺ exchange system, and Ca²⁺-ATPase may participate in the decrease of tracheal responsiveness to agonists. Reversion by PDA of the inhibitory effect of low temperature suggests a role for protein kinase C in the cooling-induced changes of tracheal responses.Correspondence to J. Cortijo at the above address