“…However, most studies of J, in the proximal tubule in vitro find 50% of this rate, i.e., 0.8-1.0 nl/mm * min (15,18,33,37 The finding of passive calcium transport in the S2 segment of the superficial proximal convoluted tubule of the rabbit is consonant with the published literature that suggests that there is an interdependence of sodium and calcium (38,39). Those maneuvers, which depress proximal tubular reabsorption of sodium and water, such as volume expansion (38), PTH, (40), insulin (41), renal vasodilators (42), and acetazolamide (43), also similarly depress proximal tubular reabsorption of calcium, which results in parallel excretion of sodium and calcium in the final urine. In contrast, those factors which dissociate sodium from calcium in the final urine enhance either distal active sodium reabsorption, such as mineralocorticoids (44, 45), or distal active calcium reabsorption, such as PTH (40) or thiazide diuretics (46).…”