DIVERSE MODULATIONS OF CHLORIDE CHANNELS IN RENAL PROXIMAL TUBULES

Cl--selective channels were detected and characterized in apical membranes of cultured rat renal proximal convoluted tubule cells (PCT) using patch-clamping methods. Subpopulations of Cl- channels modulated by cyclic nucleotides, Ca2+, or voltage were identified. Two different 30-pS, voltage-independent, Cl- channels modulated by adenosine 3',5'-cyclic monophosphate (cAMP) or Ca2+ were seen most frequently. The cAMP-dependent channel was activated by membrane-permeable analogues of cAMP, dibutyryl-cAMP or 8-bromo-cAMP. Catalytic subunit of protein kinase A (PKA) applied to detached inside-out patches, activated the channel as well, suggesting activation via phosphorylation. Channel activity was blocked by 4,4'-diisothiocyano stilbene-2,2'-disulfonic acid, by 4,4-dinitrostilbene-2,2-disulfonic acid, and by SCN-. Permeability sequence for different halides was Cl- > I > F with a Cl--to-cation permeability ratio (P-Cl/P-cation of 7:1. The Ca2+-sensitive channel was not activated by cAMP nor by PKA. A third anionic selective channel encountered infrequently is voltage dependent and has a unitary conductance of 145 pS, with a P-Cl/P-cation value of 9:1. This diversity of Cl- channels may underlie the rich repertoire of physiological functions attributed to Cl- channels.