Agonist-Mediated Changes in Intracellular pH

Abstract

Changes in intracellular pH (pHi) are likely to play an important role in regulation of vascular smooth muscle cell (VSMC) function. In most blood vessels, acidification is associated with decreased contractile tone and alkalinization with increased tone. However, the nature of agonist-mediated alterations in pH, and the role of pHi in other VSMC responses has been little studied. We have used the pH sensitive dye, BCECF, to study pH, in cultured rat aortic VSMC. Basal pHi at 37C in physiologic saline buffer (pH 7.3) was 7.08 in suspended VSMC and 7.26 in substrate-attached VSMC. An amiloride-sensitive Na+/H+ exchanger mediated pHi recovery following an acid load. Angiotensin II- and platelet-derived growth factor typified one class of VSMC agonists, causing an initial transient (less than 5 min) acidification followed by a sustained (greater than 20 min) alkalinization. The acidification phase was associated with increased Ca2+ mobilization as demonstrated by increases in intracellular Ca2+ and 45Ca2- efflux. The alkalinization was associated with Na− influx and H− efflux consistent with Na2+/H− exchange. Epidermal growth factor and phorbol esters typified another class of agonists which stimulated only a sustained alkalinization. Alterations in regulation of VSMC pHi may play an important role in VSMC hypertrophy and/or proliferation as suggested by the finding of increased cell growth and Na2+/H+ exchange in spontaneously hypertensive rat VSMC compared to Wistar–Kyoto VSMC. Although no functional correlate for initial acidification has been identified, cytoplasmic alkalinization appears to be required for the sustained formation of diacylglycerol following angiotensin II stimulation. These findings suggest that alterations in pHi may regulate several VSMC functions such as agonist-mediated signal transduction, excitation-response coupling, and growth.