Basal and hormone-activated calcium absorption in mouse renal thick ascending limbs

These studies examined transepithelial routes of basal and hormone-activated calcium absorption in thick ascending limbs of Henle's loop perfused in vitro. Single cortical (cTAL) or medullary (mTAL) thick ascending limbs were dissected from kidneys of normal mice. Tubules were perfused at 37 degrees C at pH 7.4 in bicarbonate-buffered media for cTALs and in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered solutions for mTALs. Calcium was determined by electrothermal atomic absorption spectrophotometry. External driving forces for calcium were eliminated by perfusing and bathing tubules with symmetric solutions containing identical calcium concentrations and by abolishing the transepithelial voltage (VT), by adding furosemide or bumetanide to the luminal perfusate. Basal calcium absorption averaged 60 X 10(-12) mol.s-1.cm-2 in cTAL. After addition of 1 U/ml parathyroid hormone (PTH)-(1-34), net calcium absorption increased to 162 X 10(-12) mol.s-1.cm-2 (P less than 0.05) with no change in VT. Similarly, addition of 10(-3) M dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP) to external bathing solutions increased net calcium absorption to 201 from 70 X 10(-12) mol.s-1.cm-2 in control conditions (P less than 0.025). These results demonstrate that PTH, acting at least in part through cAMP, activates a cellular pathway for calcium absorption in cTAL. In segments of mTAL, calcium absorption was strictly voltage dependent: when VT was abolished, net calcium absorption fell to values indistinguishable from zero and was not altered in the presence of dibutyryl cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)