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.
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Cattedra di Nefrologia Medica, University of Rome La Sapienza, Italy.
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Cinotti
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D000818:Animals; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D002478:Cells, Cultured; D027963:Chloride-Bicarbonate Antiporters; D004305:Dose-Response Relationship, Drug; D005920:Glomerular Mesangium / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D007477:Ions; D008564:Membrane Potentials / Q000187:drug effects; D012701:Serotonin / Q000494:pharmacology*; D015398:Signal Transduction; D012964:Sodium / Q000502:physiology; D017923:Sodium-Hydrogen Exchangers
D002352:Carrier Proteins; D027963:Chloride-Bicarbonate Antiporters; D007477:Ions; D017923:Sodium-Hydrogen Exchangers; D012701:Serotonin; D012964:Sodium; D002118:Calcium