Substrate oxidation was assessed by measuring 14CO2 production from 14C-labeled substrates in proximal convoluted tubules (PCT), medullary (MTAL), and cortical (CTAL) thick ascending limb of Henle, nephron segments rich in mitochondria and characterized by active solute transport. PCT, MTAL, and CTAL were dissected from the outer cortex, outer medulla, and the medullary rays of the cortex, respectively, of collagenase-treated rat kidney slices. Tubules were incubated at 37 degrees C in 150 microliters of Krebs-Ringer-bicarbonate buffer (pH, 7.4) with 14C-labeled substrate. 14CO2 production was linear up to 4 and 2 hours in PCT and MTAL, respectively. Freeze-thawing of the tubules markedly decreased 14CO2 production, and the addition of cyanide completely abolished it. The PCT demonstrated marked 14CO2 production from labeled succinate, 2-oxoglutarate, glutamate, glutamine, and malate (approximately 10 to 45 pmoles/mm/hr) and moderate 14CO/ production from citrate (approximately 3 pmoles/ml/hr). Little 14CO2 was released from labeled glucose and lactate in PCT. These results are consistent with the existence of gluconeogenesis in this nephron segment. By contrast, MTAL and CTAL oxidized glucose, 2-oxoglutarate, lactate, glutamate, and glutamine, but not malate, succinate, and citrate. The pentose shunt pathway accounted for approximately half of the 14CO2 produced from 1-14C glucose in MTAL and CTAL. Palmitate oxidation occurred in MTAL and CTAL but minimally in PCT. The results demonstrate a distinct pattern of substrate oxidation in PCT, MTAL, and CTAL where oxidative metabolism is critical to support active solute transport.

The effect of the state of sodium balance on the activity of the sympathetic nervous system has been evaluated previously by measuring urinary catecholamine excretion. Since urinary catecholamine may be affected by factors such as renal function or renal production of catecholamines, blood catecholamines may provide a better index of the activity of the sympathetic nervous system. The present study was undertaken to evaluate the effect of varying sodium intake on blood catecholamines.Thirteen normal subjects were studied for a period of 3 weeks in a metabolic ward. They received during the first, second, and third week 10, 100, and 200 meq sodium/day, respectively. On the seventh day of each week, when the patients had achieved sodium balance, urinary sodium excretion as well as blood levels of PRA, norepinephrine (NE), epinephrine (Ep), and dopamine (D) were measured in the supine position, at 5, 10, 15, and 20 min of upright posture, and at the end of 40 min of ambulation.The results show that: 1) blood levels of NE, Ep, and D as well as PRA were significantly higher during low sodium intake than during medium or high sodium intake, 2) as in the case of PRA, there was an inverse relationship between the blood levels of NE, Ep, and urinary sodium excretion; 3) upright posture produced a significant increment in the blood levels of NE which was not affected by sodium intake; and 4) the increment in PRA with posture was significantly greater during low sodium intake than with medium high sodium intake.The data demonstrate that: 1) the plasma levels of NE, Ep, and D are affected by the state of sodium balance, particularly during marked sodium depletion; and 2) meaningful interpretation of the significance of the blood levels of catecholamines should be made with reference to indices of sodium balance, such as urinary sodium excretion. (JClin Endocrinol Metab 48: 26, 1978) S EVERAL lines of evidence point toward the possible role of increased activity of the sympathetic nervous system in the genesis of essential hypertension (1-3). Esler et al. found hemodynamic evidence of sympathetic hyperactivity in a group of patients with high renin hypertension (4). However, Mitchell et al. (5) did not find differences in plasma catecholamine levels among hypertensive patients with different levels of PRA. The data on the relationship between urinary or blood catecholamines and essential hypertension are controversial. Some investigators have reported elevated levels of urinary catecholamines in 5-33% of patients (6-9), whereas others have found decreased excretion (10). Measurements of blood levels of catecholamines did not reveal a

Effects of parathyroid hormone (PTH), calcitonin (CT), vasopressin (VP), and glucagon (GL) on adenylate cyclase activity and cyclic AMP (cAMP) levels in isolated cortical thick ascending limbs of Henle's loop (CTAL) of the rat kidney were examined. PTH, CT, and VP each stimulated adenylate cyclase of this nephron segment in a dose-dependent manner. Stimulation of the enzyme activity was greatest with a maximal dose of PTH and least with VP. With maximal doses, the effects of PTH and CT were not additive; whether or not the effects of maximal doses of VP and PTH or CT were additive was not clear. All three hormones increase cAMP in intact CTAL ina dose-dependent manner. Maximal doses of PTH, CT, VP, and GL resulted in comparable rises in cell cAMP, and there was no additive effect. These data suggest that PTH and CT may stimulate the same adenylate cyclase moieties, whereas VP may stimulate distinct enzyme moieties, and that these three peptide hormones as well as GL definitely act on the same cell group in rat CTAL. Thus, it is possible that these hormones may induce qualitatively similar effects on CTAL functions if such effects are mediated by cAMP.

To better understand the regulation of renal gluconeogenesis that occurs in the proximal nephron, glucose production rates from various substrates were determined in defined proximal tubule segments of the rat. Tubule segments tested were the S1 and S2 segments of superficial (SF) nephrons, the S1 segments of juxtamedullary (JM) nephrons, and the S3 segments. Glucose production (in decreasing order) was: from alpha-ketoglutarate, JM S1, SF S1, SF S2; from pyruvate, SF S2, JM S1, and SF S1; from glutamine, SF S1, JM S1; and from glutamate, SF S1 = JM S1. Little glucose was produced in the S3 segments. Glucose production from glutamate was lower than that from the other three substrates in JM S1, and glutamine was the best gluconeogenic substrate in SF S1. The effects of parathyroid hormone (PTH), a known stimulator of renal gluconeogenesis, and cAMP were examined using alpha-ketoglutarate as the substrate. Both stimulated glucose production in the S1 and S2 segments of the SF nephron. Although PTH stimulated adenylate cyclase in the S1 segments of the SF and JM nephrons, it had no effect on glucose production in the JM S1. Glucose production rose in the SF S1 and JM S1 in response to increasing concentrations of hydrogen or calcium ions, indicating that gluconeogenesis can be increased in these nephron segments. Differences may therefore be present in the cellular responses to PTH distal to cAMP formation in the nephron segments of the SF and JM nephrons. These findings show the presence of both axial and internephron heterogeneity of renal gluconeogenesis and suggest the difference in the effects of PTH on the function of SF and JM nephrons.

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