pH i , [Ca 2+ ] i , and Myosin Phosphorylation in Histamine- and NH 4 + -Induced Swine Carotid Artery Contraction

Publisher Website
Google Scholar
Abstract
We examined the interaction among changes in pH i , [Ca 2+ ] i , myosin light-chain phosphorylation, and contraction in arterial smooth muscle stimulated by histamine, NH 4 + , 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 [Ca 2+ ] i . Incubation of tissues in NH 4 + increased pH i , [Ca 2+ ] i , myosin phosphorylation, and force. Washout of NH 4 + decreased pH i and transiently further increased in [Ca 2+ ] 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 [Ca 2+ ] i and force. Increasing extracellular pH coincidentally with washout of NH 4 + prevented the decrease in pH i but did not affect the NH 4 + washout-induced contraction. These data suggest that NH 4 + altered [Ca 2+ ] i and contraction by mechanisms other than its effects on pH i . The type of pH buffer did not affect the [Ca 2+ ] 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 [Ca 2+ ] i , myosin phosphorylation, and stress. Addition of 10 mmol/L NH 4 + concurrently with histamine aborted the histamine-induced decrease in pH i and significantly slowed the histamine-induced increase in [Ca 2+ ] i , myosin phosphorylation, and stress. There was little effect on histamine-induced increases in [Ca 2+ ] i , myosin phosphorylation, or contraction when three other protocols aborted the histamine-induced decrease in pH i . These data show that incubation in NH 4 + can alter [Ca 2+ ] i and contraction in both unstimulated and histamine-stimulated smooth muscle. However, these effects were not caused by NH 4 + -dependent changes in pH i . It appears that histamine-induced changes in pH i have at most minor effects on [Ca 2+ ] i and contraction of swine carotid artery.