Comparison of sodium nitroprusside and isoprenaline aerosols in histamine-induced bronchial asthma of the guinea pig

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Abstract
Sodium nitroprusside (NP) is a potent relaxant of the guinea pig's tracheal smooth muscle contracted by carbachol or histamine. At high doses, aerosols of NP delay the occurrence of symptoms of bronchial asthma in guinea pigs subjected to a histamine aerosol. However, the β-adrenergic bronchodilator isoprenaline is at least 2,600 times more potent as an anti-asthmatic than NP on a molar basis. NP and isoprenaline both reduce the blood pressure in rats when given either as infusion or as aerosol. The aerosol/infusion dosage ratio for comparable reductions of blood pressure were 1 for isoprenaline, and 10 for NP. After cessation of the application of the NP aerosol, the blood pressure rose more than twice as fast (t 1/2 of blood pressure recovery) than after cessation of the application of the isoprenaline aerosol. The poor antiasthmatic effect of NP aerosols may result from low permeability of the bronchial epithelium to the NP anion, from partial destruction in the airways or during tissue permeation, and from short half life of NP in the circulating blood. Because of its inferiority to known bronchodilators, aerosols of NP cannot be recommended for the treatment of acute bronchial asthma.