Physiological role of large, Ca2+-activated K+ channels in human glomerular mesangial cells.
1. Contraction assays and patch clamp methods were used to determine the role of K+ channels in the regulation of contractile tone of human mesangial cells (MC) in culture. 2. MC contraction was induced by vasoconstrictor agents, such as angiotensin II (AngII; 100 nmol/L) and glybenclamide (Glyb), but not by iberiotoxin (IbTX), a blocker of large Ca2+-activated K+ channels (BK(Ca)). These results suggest that Glyb-sensitive K+ channels, but not BK(Ca) channels, were active at rest. 3. In the presence of 100 nmol/L IbTX, contraction by AngII was slightly, but not significantly, enhanced, indicating that BK(Ca) has a minimal role as a negative feedback regulator of contraction. Nitroprusside (NP; 100 mu mol/L) a nitric oxide (NO) donor, atrial natriuretic peptide (ANP; 1.0 mu mol/L) and db-cGMP (10 mu mol/L) attenuated AngII-induced contraction in the absence, but not in the presence, of IbTX, suggesting that BK(Ca) channels were activated by cGMP. 4. In patch clamp experiments, three distinct K+-selective channels of 9, 65 and 150 pS (outward currents) were found in excised, inside-out patches. The 150 pS channel was completely inhibited by 100 nmol/L IbTX and displayed voltage- and calcium-dependent gating qualitatively similar to BK(Ca) in other cell types. 5. In cell attached (CA) patches, the response of BK(Ca) to bath AngII (100 nmol/L) was relatively minor in control solutions, but was considerably greater in the presence of db-cGMP. 6. In excised patches, Mg-ATP (1 mmol/L) plus db-cGMP (1 mu mol/L) activated BK(Ca) in the absence, but not the presence, of the non-specific kinase inhibitor, staurosporine. 7. Separate experiments showed that BK(Ca) were also activated by arachidonic acid and high ambient glucose concentrations. 8. These results indicate that: (i) resting MC tone is sensitive to glybenclamide and apamin; and (ii) the role of BK(Ca) as a negative feedback regulator of contraction is minimal under normal conditions but is markedly enhanced by cGMP-stimulating relaxants and arachidonic acid.
Sansom
S C
SC
Departments of Medicine and Physiology and Cell Biology, University of Texas Medical School, Houston 77030, USA.
Stockand
J D
JD
D000804:Angiotensin II / Q000494:pharmacology; D016718:Arachidonic Acid / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D002478:Cells, Cultured; D006152:Cyclic GMP / Q000494:pharmacology; D005920:Glomerular Mesangium / Q000378:metabolism* / Q000502:physiology; D005947:Glucose / Q000494:pharmacology; D006801:Humans; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*
D015221:Potassium Channels; D000804:Angiotensin II; D016718:Arachidonic Acid; D006152:Cyclic GMP; D005947:Glucose; D002118:Calcium