ABSTRACT:Cultured human hepatocytes are a valuable in vitro system for evaluating new molecular entities as inducers of cytochrome P450 (P450) enzymes. The present study summarizes data obtained from 62 preparations of cultured human hepatocytes that were treated with vehicles (saline or dimethylsulfoxide, 0.1%), ␤-naphthoflavone (33 M), phenobarbital (100 or 250 M), isoniazid (100 M) and/or rifampin (20 or 50 M), and examined for the expression of P450 enzymes based on microsomal activity toward marker substrates, or in the case of CYP2C8, the level of immunoreactive protein. The results show that CYP1A2 activity was markedly induced by ␤-naphthoflavone (on average 13-fold, n ‫؍‬ 28 preparations), and weakly induced by phenobarbital (1.9-fold, n ‫؍‬ 25) and rifampin (2.3-fold, n ‫؍‬ 22); CYP2A6 activity tended to be increased with phenobarbital (n ‫؍‬ 7) and rifampin (n ‫؍‬ 3) treatments, but the effects were not statistically significant; CYP2B6 was induced by phenobarbital (6.5-fold, n ‫؍‬ 13) and rifampin (13-fold, n ‫؍‬ 14); CYP2C8 was induced by phenobarbital (4.0-fold, n ‫؍‬ 4) and rifampin (5.2-fold, n ‫؍‬ 4); CYP2C9 was induced by phenobarbital (1.8-fold, n ‫؍‬ 14) and rifampin (3.5-fold, n ‫؍‬ 10); CYP2C19 was markedly induced by rifampin (37-fold, n ‫؍‬ 10), but relatively modestly by phenobarbital (7-fold, n ‫؍‬ 9); CYP2D6 was not significantly induced by phenobarbital (n ‫؍‬ 5) or rifampin (n ‫؍‬ 5); CYP2E1 was induced by phenobarbital (1.7-fold, n ‫؍‬ 5), rifampin (2.2-fold, n ‫؍‬ 5), and isoniazid (2.3-fold, n ‫؍‬ 5); and, CYP3A4 was induced by phenobarbital (3.3-fold, n ‫؍‬ 42) and rifampin (10-fold, n ‫؍‬ 61), but not by ␤-naphthoflavone. Based on these observations, we generalize that ␤-naphthoflavone induces CYP1A2 and isoniazid induces CYP2E1, whereas rifampin and, to a lesser extent phenobarbital, tend to significantly and consistently induce enzymes of the CYP2A, CYP2B, CYP2C, CYP2E, and CYP3A subfamilies but not the 2D subfamily. Drugs and NMEs5 are often screened for their ability to induce P450 and other drug-metabolizing enzymes with the aim of predicting or explaining drug-drug interactions and pharmacokinetic tolerance.

The patch-clamp technique was used to study the properties and the density of conducting K and Na channels in the apical membrane of rat cortical collecting tubule. The predominant K channel observed in cell-attached patches (SK channels) had an outward single-channel conductance (with LiC1 in the pipette) of 10 pS. The inward conductance (with KCI in the pipette) was 42 pS. The channel had a high open probability that increased with depolarization. Kinetic analysis indicated the presence of a single open state and two closed states. Increasing K intake by maintaining animals on a high K diet for 12-16 d increased the number of SK channels per patch by threefold (0.7-2.0/patch) over control levels. In addition, conducting Na-selective channels, which were not observed in control animals, were seen at low density (0.5/patch). These channels had properties similar to those observed when the animals were on a low Na diet, except that the mean open probability (0.84) was higher. In other experiments, the whole-cell patch clamp technique was used to measure Na channel activity (as amiloride-sensitive current, INa) and Na pump activity (as ouabain-sensitive current,/pump). In animals on a high K diet, INa was greater than in controls but much less than in rats on a low Na diet. /pump was greater after K loading than in controls or Na-depleted animals. These K diet-dependent effects were not accompanied by a significant increase in plasma aldosterone concentrations. To further investigate the relationship between K channel activity and mineralocorticoids, rats were maintained on a low Na diet to increase endogenous aldosterone secretion. Under these conditions, no increase in SK channel density was observed, although there was a large increase in the number of Na channels (to 2.7/patch). Aldosterone was also administered exogenously through osmotic minipumps. As with the low Na diet, there was no change in the density of conducting SK channels, although Na channel activity was induced. These results suggest that SK channels, Na channels and Na/K pumps are regulated during changes in K intake by factors other than aldosterone.

The activity of apical membrane Na channels in the rat cortical collecting tubule was studied during manipulation of the animals' mineralocorticoid status in vivo using a low-Na diet or the diuretic furosemide. Tubules were isolated and split open to expose the luminal membrane surface. Induction of Na channel activity was studied in cell-attached patches of the split tubules. No activity was observed with control animals on a normal diet. Channel activity could be induced by putting the animals on the low-Na diet for at least 48 h. The mean number of open channels per patch (NPo) was maximal after 1 wk on low Na. Channels were also induced within 3 h after injection of furosemide (20 mg/kg body wt per d). NPo was maximal 48 h after the first injection. In both cases, increases in NPo were primarily due to increases in the number of channels per patch (N) at a constant open probability (Po). With salt depletion or furosemide injection NPo is a saturable function of aldosterone concentration with half-maximal activity at ~ 8 nM. When animals were salt repleted after 1-2 wk of salt depletion, both plasma aldosterone and NPo fell markedly within 6 h. NPo continued to decrease over the next 14 h, while plasma aldosterone rebounded partially. Channel activity may be dissociated from aldosterone concentrations under conditions of salt repletion.

Activities of Na channels and Na pumps were studied in the rat cortical collecting tubule (CCT) during manipulation of the animals' mineralocorticold status in vivo using a low-Na diet, diuretics, or administration of exogenous aldosterone. Tubules were isolated and split open to expose the luminal membrane surface. Using the whole-cell patch-clamp technique, activities of the apical Na channels and the basolateral Na pumps were measured in principal cells as the currents inhibited by amiloride (10 I.LM) and ouabain (1 raM), respectively. Na channel current (INa) was not measurable in CCTs from control animals on a normal diet. INa was ~ 200 pA/cell in CCTs from animals on a low-Na diet or infused with aldosterone using osmotic minipumps. Currents attributable to the Na pump (/pump) were similar in control animals and animals on a low-Na diet. Maximal currents were ~ 35 pA/cell in both groups, and decreased with hyperpolarization of the cell membrane. In contrast, administration of exogenous aldosterone increased/pump fourfold. Coinfusion of aldosterone and amiloride in vivo through the minipumps did not affect the induction of INa but reduced the induction of/pump by 80%. We conclude that the induction of channel activity in this tissue is a direct action of aldosterone, whereas the induction of pump activity may be a consequence of the increased Na traffic through the epithelial ceils.

Sunitinib exhibited advantageous pharmacokinetic and dispositional properties in non-clinical species, translating into favorable properties in humans.