Effects of altered availability of Na+ on guinea-pig airway smooth muscle contractility

✍ Scribed by D. Raeburn

• Publisher: Elsevier
• Year: 1990
• Tongue: English
• Weight: 590 KB
• Volume: 3
• Category: Article
• ISSN: 0952-0600
• DOI: 10.1016/0952-0600(90)90042-h

✦ Synopsis

The effects of altering the availability of sodium ions (Na+) on contractility of the guinea-pig isolated trachealis was examined using regimens which are reported to inhibit Na+/K+ ATPase activity (ouabain), Na+/H+ exchange (amiloride, ammonium ion (NH4+)) or Na+/Ca2+ exchange (reduced extracellular Na+). Inhibition of Na+/K+ ATPase and reversal of Na+/Ca2+ exchange resulted in increased 45Ca uptake and contraction of the trachealis by voltage-sensitive and voltage-insensitive mechanisms respectively. When Na+/H+ exchange was inhibited by amiloride the tissues relaxed to below their baseline tension. The relaxation was not due to reduced Ca2+ influx. Treatment with NH4+ produced a contractile response. Reduced extracellular Na+ caused a transient contraction as a result of reversal of the normal Na+/Ca2+ exchange process leading to accumulation of Ca2+ within the cell. Since the effects of amiloride and reduced extracellular sodium were different, it is unlikely that amiloride is acting primarily by inhibiting Na+/Ca2+ exchange. Amiloride reduced tissue sensitivity to methacholine and KCl without affecting Ca2+ influx. This may involve a secondary stimulation of Na+/Ca2+ exchange following changes in [Na+]i. Ouabain also reduced tissue sensitivity to methacholine and KCl. These findings suggest that Na+ are important in determining smooth muscle contractility. If NH4+ is altering pH then, at the concentrations used, the changes in [H+] were not sufficient to alter responses to the spasmogens.