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

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 extracellul ar 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), [Ca 2+](i), myosin phosphorylation, and force. Washout of NH4+ decreased p H(i) and transiently further increased in [Ca2+](i) and force. Incubat ion of tissues in a similar concentration of Tris(+) or increasing pH( o) also increased pH(i); however, there were only modest changes in [C a2+](i) and force. Increasing extracellular pH coincidentally with was hout of NH4+ prevented the decrease in pH(i) but did not affect the NH 4+ 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 phosphory lation, or stress response to histamine stimulation. The time course o f changes in pH(i) was much slower than the time course of histamine-i nduced changes in [Ca2+](i), myosin phosphorylation, and stress. Addit ion of 10 mmol/L NH4+ concurrently with histamine aborted the histamin e-induced decrease in pal and significantly slowed the histamine-induc ed increase in [Ca2+](i), myosin phosphorylation, and stress. There wa s little effect on histamine-induced increases in [Ca2+](i), myosin ph osphorylation, or contraction when three other protocols aborted the h istamine-induced decrease in pH(i). These data show that incubation in NH4+ can alter [Ca2+](i) and contraction in both unstimulated and his tamine-stimulated smooth muscle. However, these effects were not cause d by NH4+-dependent changes in pH(i). It appears that histamine-induce d changes in pH(i) have at most minor effects on [Ca2+](i) and contrac tion of swine carotid artery.