Opioids and hibernation. II. Effects of kappa opioid U69593 on induction of hibernation in summer-active ground squirrels by "hibernation induction trigger" (HIT).

A "Hibernation Induction Trigger" (HIT) isolated from plasma of winter-hibernating woodchucks induced hibernation in summer-active ground squirrels (Citellus tridecemlineatus). Effects of kappa opioid U69593 on the HIT-induced hibernation were examined. U69593 alone did not elicit marked behavioral alteration or hibernation in summer-active ground squirrels. U69593, however, antagonized hibernation induced by HIT in summer active ground squirrels. In the guinea pig ileum myenteric plexus-longitudinal muscle preparation, woodchuck HIT depressed the electrically-induced contraction. The depression was, however, neither reversed nor blocked by naloxone even when naloxone was used at high doses. This study demonstrates that kappa opioid, at least in the case of U69593, was unable to induce hibernation in the summer-active ground squirrels. The results also demonstrate that woodchuck HIT, like the bear HIT, did not act directly at opioid receptors. Together with our previous observation that naloxone blocked summer hibernation induced by HIT (Bruce et al., Life Sci.., this issue), it is tempting to suggest that HIT may not mediate its effects through kappa opioid receptors but may do so through other types of opioid receptors such as mu or delta. U69593 may antagonize HIT-induced hibernation as a mu or delta receptor antagonist.

Peritubular transport of ochratoxin A by single rabbit renal proximal tubules.

Epifluorescence microscopy was used to study peritubular transport of the fluorescent mycotoxin ochratoxin A (OTA) into single proximal tubule segments of the rabbit. Initial rates of OTA uptake into S2 segments were saturable and adequately described by Michaelis-Menten kinetics, with an apparent Km of 2.2+/-0.3 microM (SEM). Several lines of evidence indicated that peritubular uptake of OTA in S2 segments was effectively limited to the "classical" organic anion transporter. First, 5 mM p-aminohippurate (PAH) cis-inhibited the uptake of 1 microM OTA into tubules by 96%. Kinetic analysis of the inhibition of OTA uptake by PAH (100 microM to 5 mM) yielded an apparent Ki of 164 microM, similar to the 100 to 200 microM range of Km values previously reported for the peritubular uptake of PAH. Second, efflux of OTA from tubules was trans-stimulated 3.2-fold by the presence of 2.5 mM PAH in the uptake medium. Third, 100 microM alpha-ketoglutarate (alphaKG) trans-stimulated the uptake rate of 1 microM OTA by 1.8-fold. Fourth, besides PAH, other organic anions effectively cis-inhibited the uptake of 1 microM OTA into tubules (inhibitor, % inhibition): 1.5 mM alphaKG, 80%; 1 mM probenecid, 100%; 1 mM piroxicam, 100%; 1 mM octanoate, 100%. In contrast, 1.5 mM tetraethylammonium, an organic cation, blocked uptake of 1 microM OTA by only 7%. The inhibition of OTA uptake into S1 and S3 segments of the proximal tubule was qualitatively similar: 5 mM PAH cis-inhibited the uptake of 1 microM OTA by approximately 95% in both S1 and S3 segments. Thus, peritubular OTA uptake into all segments of the proximal tubule appears to be dominated by its interaction with the classical organic anion transporter. The high-affinity and relatively high capacity of this pathway for OTA suggest that peritubular uptake may be a significant avenue for the entry of this toxin into proximal tubule cells.

Effect of alpha-ketoglutarate on organic anion transport in single rabbit renal proximal tubules.

The effect of exogenous alpha-ketoglutarate (alpha KG) and the peritubular Na(+)-dicarboxylate (Na-DC) cotransporter on organic anion/dicarboxylate (OA/DC) exchange in S2 segments of single, nonperfused rabbit proximal tubules was measured using 1 microM fluorescein (FL), a model OA, and epifluorescence microscopy. The effect of different transmembrane distributions of 10 microM alpha KG on peritubular FL uptake was measured at 37 degrees C using bicarbonate-buffered, nutrient-containing buffers, which are conditions similar to those found in vivo. Compared with FL uptake in the absence of exogenous alpha KG, preloading tubules with alpha KG (trans-configuration) or acute exposure to alpha KG (cis-configuration) increased FL uptake 62% and 54%, respectively, whereas a cis-trans-configuration of alpha KG increased FL uptake by 76%. The cis-stimulation of FL uptake by alpha KG was rapid, within 5-7 s. This stimulation was blocked 96% by simultaneous exposure to 2 mM Li+, indicating that stimulation of transport was secondary to the uptake of exogenous alpha KG. In the absence of exogenous alpha KG, selective inhibition of Na-DC cotransport using 2 mM Li+ or 1 mM methylsuccinate decreased FL uptake by 25% (effects that were reversible but not additive), suggesting that the Na-DC cotransporter recycles endogenous alpha KG that has left the cell in exchange for FL and that this activity supports approximately 25% of baseline activity of the OA/DC exchanger. With recycling of alpha KG accounting for approximately 25% of FL uptake and with accumulation of exogenous alpha KG accounting for another approximately 75% increase in FL uptake, Na-DC cotransport appears to directly support (25% + 75%)/175%, or approximately 57%, of total FL transport.

Relation of cysteine conjugate nephrotoxicity to transport by the basolateral organic anion transport system in isolated S2 segments of rabbit proximal renal tubules.

We examined basolateral transport of the radiolabeled zwitterionic nephrotoxic cysteine S-conjugate, S-(1,2-dichlorovinyl)-L-cysteine (DCVC), inhibition of such transport and the effects of inhibition of transport on the toxicity produced by DCVC in isolated S2 segments of rabbit proximal tubules. High concentrations of unlabeled DCVC itself and an unlabeled nontoxic cysteine S-conjugate, S-(2-benzothiazole)-L-cysteine cis-inhibited the basolateral uptake of radiolabeled DCVC by approximately 80 to 85%. High concentrations of para-aminohippurate, the prototype substrate for the basolateral organic anion transport system, and probenecid, a well-known inhibitor of basolateral organic anion transport, cis-inhibited the basolateral uptake of radiolabeled DCVC by approximately 70%, whereas a high concentration of L-phenylalanine had little effect. High concentrations of S-(2-benzothiazole)-L-cysteine and para-aminohippurate in the bathing medium with DCVC inhibited the loss of 86Rb (used as a K+ surrogate to measure toxicity) from S2 segments produced by DCVC alone to approximately the same extent as they inhibited uptake of DCVC. Under the same circumstances, probenecid completely inhibited 86Rb loss. These data indicate that in rabbit proximal renal S2 tubules basolateral entry of DCVC can occur to a major extent via the organic anion transport pathway and that inhibition of such entry can reduce toxicity to approximately the same extent that entry is reduced. They also suggest that probenecid provides additional protection from DCVC toxicity.