Serotonin and the Glomerular Mesangium Mechanisms of Intracellular Signaling

Serotonin, a release product of activated platelets, stimulates proliferation and prostaglandin synthesis in cultured smooth muscle-like glomerular rat mesangial cells by activation of phospholipase and protein kinase C. To further characterize the signaling mechanisms used by serotonin, we monitored its effects on intracellular free Ca2+, pH, and membrane potential of cultured rat mesangial cells with sensitive fluorometric techniques. Activation of a 5-HT2 receptor, blocked by the specific receptor antagonists ketanserin and ritanserin, triggered immediate discharge of intracellular Ca2+ stores. The resulting rise of cytosolic free Ca2+ was accompanied by simultaneous membrane depolarization and followed within 30-60 seconds by prolonged cytosolic alkalinization. Depolarization and cytosolic free Ca2+ elevation were persistent in the continued presence of serotonin and were rapidly reversed by competitive receptor displacement with ketanserin or ritanserin. Depolarization is secondary to enhanced Cl− conductance, whereas it is relatively independent of Na+, K+, and Ca2+ fluxes. The putative Cl− channel is regulated by Ca2+ since ionomycin and other stimuli of cytosolic free Ca2+ mimic the effects of serotonin on membrane potential, whereas serotonin-induced depolarization is blunted in cells pretreated with the intracellular Ca2+ chelator BAPTA. Cytosolic alkalinization occurs in HCO3−free solutions resulting from enhanced activity of a Na+-H+ exchanger and blocked by extracellular Na+ removal or amiloride. In the presence of HCO3−, serotonin elicits a persistent acidification, revealing simultaneous enhancement of a Na+-independent Cl−-HCO3−" countertransport. These findings indicate multiple pathways for contraction and long-term functional changes induced by serotonin in mesangial cells, with potential relevance to glomerular and systemic hypertension.