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(Hypertension. 1995;25:482-489.)
© 1995 American Heart Association, Inc.

Articles

pH_i, [Ca²⁺]_i, and Myosin Phosphorylation in Histamine- and NH₄⁺-Induced Swine Carotid Artery Contraction

Xiao-Liang Chen; Christopher M. Rembold

From the Cardiovascular Division, Departments of Internal Medicine and Physiology, University of Virginia Health Sciences Center, Charlottesville.

Abstract We examined the interaction among changes in pH_i, [Ca²⁺]_i, myosin light-chain phosphorylation, and contraction in arterial smooth muscle stimulated by histamine, NH₄⁺, Tris⁺, and/or changes in extracellular pH (pH_o). We loaded swine carotid medial tissues with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein to measure pH_i or aequorin to measure [Ca²⁺]_i. Incubation of tissues in NH₄⁺ increased pH_i, [Ca²⁺]_i, myosin phosphorylation, and force. Washout of NH₄⁺ decreased pH_i and transiently further increased in [Ca²⁺]_i and force. Incubation of tissues in a similar concentration of Tris⁺ or increasing pH_o also increased pH_i; however, there were only modest changes in [Ca²⁺]_i and force. Increasing extracellular pH coincidentally with washout of NH₄⁺ prevented the decrease in pH_i but did not affect the NH₄⁺ washout-induced contraction. These data suggest that NH₄⁺ altered [Ca²⁺]_i and contraction by mechanisms other than its effects on pH_i. The type of pH buffer did not affect the [Ca²⁺]_i, myosin phosphorylation, or stress response to histamine stimulation. The time course of changes in pH_i was much slower than the time course of histamine-induced changes in [Ca²⁺]_i, myosin phosphorylation, and stress. Addition of 10 mmol/L NH₄⁺ concurrently with histamine aborted the histamine-induced decrease in pH_i and significantly slowed the histamine-induced increase in [Ca²⁺]_i, myosin phosphorylation, and stress. There was little effect on histamine-induced increases in [Ca²⁺]_i, myosin phosphorylation, or contraction when three other protocols aborted the histamine-induced decrease in pH_i. These data show that incubation in NH₄⁺ can alter [Ca²⁺]_i and contraction in both unstimulated and histamine-stimulated smooth muscle. However, these effects were not caused by NH₄⁺-dependent changes in pH_i. It appears that histamine-induced changes in pH_i have at most minor effects on [Ca²⁺]_i and contraction of swine carotid artery.

Key Words: aequorin • ammonium chloride • calcium • hydrogen-ion concentration • phosphorylation • muscle, smooth, vascular

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