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Human Podocytes Possess a Stretch-Sensitive, Ca²⁺-Activated K⁺ Channel: Potential Implications for the Control of Glomerular Filtration

Michael J. Morton^*, Katie Hutchinson^*, Peter W. Mathieson, Ian R. Witherden, Moin A. Saleem and Malcolm Hunter^*

*School of Biomedical Sciences, University of Leeds, Leeds; and Academic Renal Unit, University of Bristol, Southmead Hospital, Bristol, United Kingdom

Correspondence to Dr. Malcolm Hunter, School of Biomedical Sciences, Worsley Building, University of Leeds, Leeds LS2 9NQ, UK. Phone: 44-133-3434240; Fax: 44-133-3434228; E-mail: m.hunter{at}leeds.ac.uk

Podocytes express many proteins characteristic of smooth muscle, such as actin and myosin. They also express receptors to several vasoactive agents, including acetylcholine and angiotensin II; these phenotypic properties suggest that podocytes are not static entities but may respond to physiologic stimuli. The electrophysiologic properties of a conditionally immortalized human podocyte cell line that expresses the specific podocyte proteins nephrin, podocin, and synaptopodin were examined by patch clamp. Channels that were highly K⁺-selective and had a conductance of 224 ± 11.5 pS in symmetrical 150 mM K⁺ solutions were identified. Channel activity was Ca²⁺- and voltage-dependent, being increased with an increase in Ca²⁺ or depolarization, and inhibited by penitrem A. The conductance and voltage- and Ca²⁺-dependence suggest that this is the large-conductance calcium-activated K⁺ channel, BK (KCNMA1)—this was supported by reverse transcription-PCR experiments that showed the presence of the BK encoding mRNA, along with expression of KCNMB subunit types 3 and 4. In sections of human glomeruli, immunocytochemistry revealed that BK co-localizes with the podocyte-specific protein nephrin, indicating that these channels are present in native human podocytes. In whole-cell experiments, penitrem A inhibited outward currents to the same extent as tetra-ethyl ammonium (TEA) but did not affect the membrane potential. Channel activity was also increased by applying suction to the patch pipette or by dilution of the bathing medium, indicating that these channels are stretch sensitive. Thus, these channels do not contribute to the resting membrane potential but are activated by a rise in intracellular Ca²⁺, membrane depolarization, cell swelling, or membrane stretch. By implication, these results suggest that podocytes may be able to respond to changes in the glomerular capillary pressure and modulate the GFR.

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