PH(I), [CA2+](I), AND MYOSIN PHOSPHORYLATION IN HISTAMINE-INDUCED AND NH4+-INDUCED SWINE CAROTID-ARTERY CONTRACTION

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 pal 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.