Interrelationship between growth factor-induced pH changes and intracellular Ca2+

Abstract

Many mitogens cause rapid changes in intracellular pH and Ca2+. We studied the patterns of pH and Ca2+ changes after exposure of murine fibroblasts to platelet-derived growth factor (PDGF), bombesin, phorbol 12-myristate 13-acetate (PMA), and the vasoactive peptide bradykinin. Intracellular pH and Ca2+ were measured by using the fluorescent dyes 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and fura-2. Three distinct patterns of intracellular pH change were observed. PDGF and bombesin caused a rapid (maximum change, less than 2 min) cytoplasmic acidification of 0.03 pH unit followed by a slower (5-10 min) alkalinization of approximately 0.11 pH unit above the resting pH of 6.88. PMA caused alkalinization without causing the early acidification. Bradykinin caused rapid acidification without the slower net alkalinization. Ionomycin also caused acidification without subsequent alkalinization. All acidification responses were amiloride resistant. Patterns of intracellular Ca2+ response were also determined for each agent. PDGF and bombesin caused a transient increase in cytoplasmic Ca2+ from a resting level of 85 +/- 12 nM to 190 +/- 12 nM within 2 min and return to baseline within 5 min. PMA caused no change in intracellular Ca2+. Bradykinin caused the most rapid (maximum response, less than 20 sec) increase in intracellular Ca2+. For each agonist, the Ca2+ transient could be blocked by buffering intracellular Ca2+ with quin-2. In Ca2+-buffered cells, PDGF, bombesin, bradykinin, and ionomycin failed to induce cellular acidification, but alkalinization responses to PDGF, bombesin, and PMA persisted. We propose that the transient acidification seen with PDGF, bombesin, and other agents is the result of increased intracellular Ca2+. However, growth factor-induced alkalinization via the Na+/H+ exchanger is independent of changes in Ca2+.