Genetically correlated leaf tensile and morphological traits are driven by growing season length in a widespread perennial grass.

PREMISE: Leaf tensile resistance, a leaf's ability to withstand pulling forces, is an important determinant of plant ecological strategies. One potential driver of leaf tensile resistance is growing season length. When growing seasons are long, strong leaves, which often require more time and resources to construct than weak leaves, may be more advantageous than when growing seasons are short. Growing season length and other ecological conditions may also impact the morphological traits that underlie leaf tensile resistance. METHODS: To understand variation in leaf tensile resistance, we measured size-dependent leaf strength and size-independent leaf toughness in diverse genotypes of the widespread perennial grass Panicum virgatum (switchgrass) in a common garden. We then used quantitative genetic approaches to estimate the heritability of leaf tensile resistance and whether there were genetic correlations between leaf tensile resistance and other morphological traits. RESULTS: Leaf tensile resistance was positively associated with aboveground biomass (a proxy for fitness). Moreover, both measures of leaf tensile resistance exhibited high heritability and were positively genetically correlated with leaf lamina thickness and leaf mass per area (LMA). Leaf tensile resistance also increased with the growing season length in the habitat of origin, and this effect was mediated by both LMA and leaf thickness. CONCLUSIONS: Differences in growing season length may promote selection for different leaf lifespans and may explain existing variation in leaf tensile resistance in P. virgatum. In addition, the high heritability of leaf tensile resistance suggests that P. virgatum will be able to respond to climate change as growing seasons lengthen.

Effects of prior oral contraceptive use and soy isoflavonoids on estrogen-metabolizing cytochrome P450 enzymes.

Estrogen exposure and metabolism may play an important role in the development of estrogen-sensitive cancers in postmenopausal women. In this study we investigated whether past oral contraceptive (OC) administration or current dietary isoflavonoids (IF) affected expression and/or activity of steroid hormone-metabolizing cytochrome P450 (CYP) enzymes using complementary primate and cell culture models. One-hundred-eighty-one female cynomolgus macaques were randomized to receive OC or nothing for 26 months premenopausally, then ovariectomized and randomized to one of three diets for 36 months: an IF-depleted soy protein isolate (Soy-) diet, a Soy diet with IF (Soy+), or a Soy- diet supplemented with conjugated equine estrogens (CEE). Prior OC-treatment significantly reduced CYP gene expression in the mammary gland (< or =60% of OC-). Dietary IFs had no effect on CYP expression, while CEE-treatment decreased CYP1A1 and increased CYP3A4 mRNA in a tissue-specific manner. For in vitro studies, we measured effects of the isoflavonoids genistein, daidzein and equol on CYP activity using intact V79 cells stably transfected to express CYP1A1, CYP1B1, or CYP3A4. All three IFs significantly altered CYP activity in a dose-dependent and isoform-specific manner (20-95% inhibition versus controls). These results suggest potential mechanisms for prior OC and dietary IF effects on cancer risk in estrogen-responsive tissues.

Stabilization of the anticodon stem-loop of tRNALys,3 by an A+-C base-pair and by pseudouridine.

NMR spectroscopy was used to determine the solution structures of RNA oligonucleotides comprising the anticodon domain of tRNALys,3. The structural effects of the pseudouridine modification at position 39 were investigated and are well correlated with changes in thermodynamic parameters derived from temperature dependent UV measurements. The pseudouridine-containing hairpin is thermodynamically more stable than the unmodified hairpin by 5 degreesC, and this corresponds with increased base stacking on the 3' side of the tRNA anticodon loop. An A+38-C32 base-pair also forms at the base of the anticodon stem with an approximate pKa of 6 for A38. Formation of the A+-C base-pair increases the Tm of both pseudouridine modified and unmodified RNA hairpins by 5-6 degreesC, and decreases the DeltaG degrees for hairpin formation by 1 kcal/mol. Solution structures were determined for both psi39 and unmodified hairpins under limiting pH conditions at pH 5 and pH 7 to assess the structural effects of both psi modification and the additional A+-C base-pair on tRNALys,3 structure. The A+38-C32 base-pair strengthens the 31-39 base-pair, and induces formation of a dynamic U33-A37 base-pair that effectively reduces the normal seven nucleotide anticodon loop to a three nucleotide UUU loop. These undermodified tRNALys,3 anticodon loops are distinctly different from those seen for other tRNAs exemplified by tRNAPhe. The conformation of the tRNA loop has important implications for the role of nucleoside modification in codon-anticodon recognition and for utilization of tRNALys,3 by HIV-1 as the native reverse transcriptase primer.

Lupus erythematosus-like disease due to hydrazine.

A case of systemic lupus erythematosus-like disease due to occupational exposure to hydrazine is described. The patient had four of the 1982 revised criteria for SLE (malar rash, photosensitivity, antinuclear antibody, and antibody to nDNA) and genetically is a slow acetylator with the HLA DR2,3 phenotype. Many of her healthy family members had antibodies to nuclear constituents. Lymphocytes from the patient and an identical twin sister, but not from three normal control subjects, showed inhibition of pokeweed mitogen-stimulated IgG synthesis after five daily exposures of each subject to hydrazine. Chemicals such as hydrazine in the environment can induce cases of SLE-like disease in predisposed persons.

Pharmacological analysis of agonist-antagonist interactions at acetylcholine muscarinic receptors in a new urinary bladder assay.

1. Agonist-antagonist interactions at acetylcholine (ACh) muscarinic receptors have been analysed by use of an improved urinary bladder assay, isolated and intact, from the mouse. With 5-methylfurmethide as agonist, validated cumulative concentration-effect curves were obtained in less than 7 min by re-dosing before the response plateaux began to fade. 2. The pKB value estimated for pirenzepine was 6.76. The pKB values estimated for atropine and N-methylatropine from data obtained at concentrations which produced dose-ratios greater than 20 and 60 were 8.90 and 9.58, respectively. 3. The deviation from simple competitive behaviour at low dose-ratios with atropine and N-methylatropine was consistent with the operation of saturable antagonist removal processes. The deviation observed with atropine was corrected by pre-incubation with methylbutyrate, an alternative substrate for 'atropine esterase'. 4. The simple competitive behaviour of N-methylatropine was restored following pre-incubation with the neuronal choline uptake blocker hemicholinium-3 (HC-3). However, the pKB estimated for N-methylatropine under these conditions was low. This latter result could be accounted for by the observed behaviour of HC-3 as a competitive antagonist of ACh muscarinic receptors (pKB = 4.01). 5. We conclude that the modified mouse urinary bladder assay is suitable for the quantitative analysis of muscarinic receptor interactions. In addition, we postulate the existence of a previously undescribed uptake mechanism for quaternary muscarinic receptor antagonists.

Comparative studies on fatty acid synthesis, glycogen metabolism, and gluconeogenesis by hepatocytes isolated from lean and obese Zucker rats.

Hepatocytes isolated from genetically obese female Zucker rats and lean female Zucker rats were compared. Hepatocytes from fed obese rats exhibited greater rates of fatty acid synthesis, more extensive accumulation of lactate and pyruvate from their glycogen stores, increased rates of net glucose utilization but produced less ketone bodies from exogenous fatty acids and had lower citrate levels than hepatocytes from lean rats. Lipogenesis was not as sensitive to dibutyryl cyclic AMP (DBcAMP) inhibition in hepatocytes from obese rats but glycogenolysis was stimulated to the same extent by this nucleotide in both preparations. Ketogenesis was less sensitive to stimulation by DBcAMP in hepatocytes from obese rats. A difference in sensitivity of lipogenesis to DBcAMP was not found when lactate plus pyruvate was added to the incubation medium, suggesting that a greater rate of glycolysis by hepatocytes from obese rats accounts for their relative insensitivity to DBcAMP. Citrate levels were elevated by DBcAMP to a greater extent in hepatocytes from obese rats. Hepatocytes prepared from lean rats starved for 48 hr were glycogen depleted and lacked significant capacity for lipogenesis and glycogen synthesis. In contrast, hepatocytes isolated from starved obese rats retained considerable amounts of liver glycogen and exhibited detectable rates of lipogenesis and glycogen synthesis. Hepatocytes prepared from starved lean rats gave faster apparent rates of lactate gluconeogenesis than hepatocytes prepared from starved obese rats. Thus, hepatocytes prepared from obese Zucker rats are more glycogenic, glycolytic, and lipogenic but less ketogenic and glucogenic than hepatocytes prepared from lean rats.

Micturition in the unanesthetized rat: effects of intrathecal capsaicin, N-vanillylnonanamide, 6-hydroxydopamine and 5,6-dihydroxytryptamine.

Unanesthetized rats chronically implanted with vesical and intrathecal catheters were injected intrathecally (i.t.) with either capsaicin (CAP), N-vanillylnonanamide (VN), 6-hydroxydopamine (6-OHDA), or 5,6-dihydroxytryptamine (5,6-DHT). The volume-evoked micturition reflex was assessed by cystometrography before, and 2 h, 1 day and 7 days after injection. In control and vehicle-injected rats, the infusion of saline into the bladder resulted in a periodic contraction of the bladder with synergic sphincter relaxation. One day after i.t. CAP and VN (70 micrograms each), 50% and 30% of the animals displayed a blockade of the micturition reflex, respectively. In these animals, the infusion of saline resulted in a gradual rise in bladder pressure up to a plateau (overflow pressure) equivalent to the predrug bladder opening pressure. When the plateau was reached, continuous dribbling of urine with no bladder contractions was observed. Most of the affected rats displayed some micturition responses by day 7. Following i.t. injection of 5,6-DHT (20 micrograms) or 6-OHDA (20 micrograms), the micturition reflex displayed small but significant increases in bladder volume with no changes in pressure profile. Small primary afferents, sensitive to the neurotoxic effects of CAP and VN appear to play a major tonic role in the regulation of the micturition reflex in unanesthetized rats. Serotonergic and adrenergic descending pathways might play a role in the maintenance of resting bladder tone.

Regulation of the formation of stable adducts between acetaldehyde and blood proteins.

Recent reports have described increased levels of a fast-moving hemoglobin (Hb) fraction in alcoholic patients and formation in vitro of stable adducts between acetaldehyde and Hb as well as between acetaldehyde and albumin. In the present study, we have found that factors other than acetaldehyde concentration can influence the rate of stable adduct formation. HbAo was purified and its 2,3-diPglycerate removed by dialysis. Under anaerobic condition and at 37 degrees, acetaldehyde (5 microM) reacted with deoxyHbAo to form 0.26 +/- 0.02 (+/- SEM) nmol of stable adduct/149 nmol Hb in 2 hr. By comparison, acetaldehyde reacted more slowly with oxyHbAo and carbonylHbAo; the rates were 0.21 +/- 0.01 (p less than 0.001) and 0.18 +/- nmol/149 nmol Hb/2 hr (p less than 0.001), respectively. Pyridoxal 5'-phosphate (50-500 microM), under anaerobic condition, inhibited by 36-56 percent the irreversible binding of acetaldehyde to deoxyHbAo. Ascorbic acid (2.5-10 mM) increased by 31-46 percent (p less than 0.001) the irreversible binding of acetaldehyde to human serum albumin and by 8-10 percent (p less than 0.05) the irreversible reaction of acetaldehyde with serum proteins. We conclude that the nonenzymatic binding of acetaldehyde to Hb, human serum albumin and serum proteins is influenced by factors other than acetaldehyde concentration. These factors include oxygen tension, pyridoxal 5'-phosphate and ascorbic acid. Among these factors, oxygen tension may be the most important in vivo.

A new rabbit species (Sylvilagus, Mammalia: Leporidae) from the lowlands of Venezuela.

A new species of Venezuelan rabbit of the genus Sylvilagus from Fundo Millano (08 degrees 46'N and 69 degrees 56'W) and Chorrosco Bajo (08 degrees 05'N and 69 degrees 18'W), between 190 and 120 masl, state of Barinas, is described based on: 1. Body and skull measurements. 2. Coloration patterns of the pelage. 3. Arrangement and length of the color hair bands of dorsal, lateral, ventral nuchal, and gular patches. Body and cranial measurements, and some color patterns of the new species, Sylvilagus varynaensis, were compared with those of the closest relative groups such as S. brasiliensis (from Venezuela and Brazil), S. b. meridensis from the Venezuelan paramos, and three of the most representative groups of S. floridanus (S. f. continentis, S. f. orinoci, and S. f. valenciae). Most of the values recorded for these parameters were significantly higher for the new species (P < 0.005; Student "t" test). Cluster and principal components analysis of the data recorded for cranial characteristics indicated that S. varynaensis is the largest and darkest of the known Venezuelan rabbits, with a broader elongated skull and a different arrangement of the color hair bands.

Epidural injections of bupivacaine, morphine, fentanyl, lofentanil, and DADL in chronically implanted rats: a pharmacologic and pathologic study.

A new technique of epidural catheterization in rats is described. The pharmacologic characterizations of the model were established after epidural injection of bupivacaine, morphine, fentanyl, lofentanil, and D-Ala2-D-Leu5-enkephalin (DADL) on hot plate (HP) and tail flick (TF). In addition, rostral spread, motor function, behavior, and reproducibility of the effects over time were assessed. The time-response curves showed an almost immediate onset of action for bupivacaine, fentanyl, and lofentanil and a delayed onset for morphine and DADL. Morphine and lofentanil displayed a significantly longer duration of action than bupivacaine, fentanyl, and DADL. The dose-response curves were monotonic and the slopes were log-linear. Based on the ED50 values, the following rank order of potency was obtained 1 day after catheterization for both HP and TF: lofentanil much greater than fentanyl greater than morphine much greater than DADL greater than bupivacaine. Intraperitoneal (IP) administration of naloxone antagonizes the agonist effects of epidural morphine, fentanyl, and lofentanil. To assess the role in analgesia played by epidural vascular uptake after epidural administration of morphine, fentanyl, and lofentanil, the lowest maximally effective epidural dose of these agents was given intravenously. After iv fentanyl and lofentanil, the analgesic and behavioral effects were not different from the values obtained after epidural administration. By contrast, the effects were negligible after iv morphine when compared with the epidural route. Epidural vascular uptake is thought to be low for morphine and high for fentanyl and lofentanil. The reproducibility of the analgesic and behavioral effects over time was assessed by epidurally injecting the lowest maximally effective dose of bupivacaine, morphine, fentanyl, and lofentanil 1 day and 10 days after catheterization. After 10 days, a significant reduction of analgesic and behavioral effects was noted and was thought to be due to a complete fibrotic sheath surrounding the epidural catheter.

Bronchospasm and hypotension during cardiopulmonary bypass after preoperative cimetidine and labetalol therapy.

A 64-yr-old asthmatic patient underwent a two-vessel aortocoronary vein grafting. Before surgery, the patient received cimetidine 400 mg and labetalol 650 mg. During the first 60 min of bypass, hypotension (40-45 mm Hg) was observed in spite of phenylephrine 14 mg. This initial hypotension was followed, during rewarming, by a slow increase in arterial pressure to 150 mm Hg. On cessation of bypass, bronchospasm was observed and was protracted. It is assumed that labetalol clearance and metabolism were reduced by cimetidine, that labetalol alpha-antagonism was responsible for the vasodilatation withstanding the phenylephrine, and that a combination of labetalol beta-antagonism and phenylephrine alpha-agonism initiated the bronchospasm. These observations indicate that, after labetalol therapy, higher doses of vasopressor agents such as phenylephrine may be necessary, but that such therapy may lead to bronchospasm in asthmatic patients.

Drug effects on urinary bladder tone during spinal morphine-induced inhibition of the micturition reflex in unanesthetized rats.

In an unanesthetized chronic rat model involving the placement of one catheter in the bladder for cystometrography and one catheter in the intrathecal (IT) space for drug injections, 10 micrograms morphine sulfate injected intrathecally (it) produced long-lasting inhibition of the volume-evoked micturition reflex. During inhibition of the micturition reflex, intravesical pressure rose with infusion until a continuous emission of urine (dribbling) was observed. Such a level of intravesical pressure (overflow pressure) was significantly greater than the premorphine bladder opening pressure (+56%). During dribbling, no periodic vesical contractions were observed, and the effects of a variety of agents on bladder tone were assessed. Significant (P less than 0.05) increases in vesical pressure over that produced by morphine were observed after intraperitoneal (ip) injection of carbachol (+86%), bethanechol (+55%), norepinephrine (+53%), methoxamine (+88%), and ST-91, an alpha 2-adrenergic agonist (+70%). The increased vesical pressure was not accompanied by an increase in the rate of urine expression. Intraperitoneal injection of serotonin produced no effects on intravesical pressure or urine expression. Significant decreases in the otherwise elevated intravesical pressure were observed after ip injection of isoproterenol (-30%) and phentolamine (-21%), with no change in the rate of urine expression. In contrast, ip injection of apomorphine (dopamine agonist) resulted in significant decreases in vesical pressure (-49%) and near maximal emptying of the bladder. Apomorphine produced no effects on it morphine-induced antinociception as assessed by the tail flick response. Regarding potential treatments of spinal morphine-induced urinary retention, the present study suggests that: 1) cholinomimetic and alpha-adrenergic agonist agents might be harmful; 2) beta-adrenergic agonist and alpha-adrenergic blocking agents might be useful; and 3) dopaminergic agonist agents might be the drugs of choice.

Distribution in cerebrospinal fluid, blood, and lymph of epidurally injected morphine and inulin in dogs.

We describe procedures for catheterizing the epidural space, the azygos vein, and the thoracic lymph duct of dogs without using fluoroscopy. The success rates of the procedures were 100, 80, and 50%, respectively (n = 10). To assess the validity of the model, 3H-morphine and unlabeled morphine (2 mg) were injected epidurally in ten dogs. Lumbar cerebrospinal fluid (CSF), azygos venous blood, arterial blood, and lymph were sampled before and 5, 20, 60, 120, 180, 240, 300 and 360 min after injection. During the first 20 min, morphine levels in the azygos vein were about three and ten times greater than arterial and lymphatic levels, respectively (n = 3; P less than 0.01). Morphine levels were significantly greater in the azygos vein (n = 8) and the femoral artery (n = 10) during the first 20 and 60 min than they were later, respectively (P less than 0.05). In the lymph (n = 5), the levels of morphine at 60 min were statistically greater (P less than 0.05) than levels at 4, 5, and 6 hr. At no time were the concurrent arterial and lymph levels different from each other. In the lumbar CSF, the morphine peak concentration was reached 5-60 min after epidural injection and ranged between 5 and 93 micrograms/ml. In the CSF, the levels of morphine were significantly greater during the first 20 min than later (n = 7; P less than 0.05). The washout of the lumbar CSF curve for morphine appeared to be fitted by a two-compartment open model. The t1/2-alpha and t1/2-beta values were 14.7 +/- 7.2 min and 106 +/- 45 min, respectively (mean +/- SD). Cumulative percentages of the epidural dose of morphine passed into the azygos system within the first 5, 20, 60, and 120 min after injection were calculated to be 4.0 +/- 2.1, 23.5 +/- 14.6, 49.2 +/- 34.2, and 55.9 +/- 35.3, respectively (mean +/- SD; n = 8). Both 14C-inulin and 3H-morphine were injected epidurally in one dog and intrathecally in another dog. In the CSF, morphine appears to be cleared at a rate similar to that of inulin. The fraction of morphine and inulin crossing the dura after epidural injection was calculated to be 0.31% and 0.59%, respectively.

The effect of pseudouridine and pH on the structure and dynamics of the anticodon stem-loop of tRNA(Lys,3).

The anticodon stem-loop hairpin of tRNA(Lys,3) was synthesized and the solution structure determined by NMR spectroscopy. The hairpin is thermodynamically stabilized by pseudouridine as determined by UV Tm measurements, and the local loop structure is stabilized with base-stacking of the nucleosides in the anticodon loop 5' of the psi 39 nucleoside modification. The tRNA(Lys,3) hairpin also contains an A(+)-C base-pair that effectively reduces the size of the normal 7 nucleotide anticodon loop to 5 nucleotides and induces a change in the loop backbone conformation. The stabilizing effects of the A(+)-C base-pair and pseudouridine are only partially additive, suggesting that the conformational changes caused by each modification are not completely compatible. The structure of the anticodon loop is distinctly different from that seen for other tRNAs exemplified by tRNA(Phe), suggesting that the full complement of modified nucleosides present in tRNA(Lys,3) should significantly change the structure compared to the unmodified tRNA anticodon loop. The conformation of the loop has important implications for the role of nucleoside modification in codon-anticodon recognition and for utilization of tRNA(Lys,3) by HIV-1 as the natural reverse transcriptase primer.

Genetic diversity of Pneumocystis carinii derived from infected rats, mice, ferrets, and cell cultures.

The degree of strain and/or species diversity among Pneumocystis carinii isolates is unknown. As a first approach to the study of P. carinii genetic relatedness, we compared the pulsed field gel electrophoretic karyotypes of P. carinii derived from lung homogenates of three immunosuppressed host animals: rats transtracheally inoculated with P. carinii-infected mouse lung; and ferrets which developed reactivated latent P. carinii pneumonia. Rat P. carinii propagated on HEL299 cells was also examined. Karyotypes of P. carinii DNA from both rat lung homogenate and cell culture were identical (14 bands, 315-680 kb). In contrast, mouse and ferret P. carinii DNA karyotypes were each distinctly different from the rat P. carinii samples (mouse P. carinii 15 bands, 315-610 kb; ferret P. carinii nine bands, 410-760 kb). Three distinct rat P. carinii gene probes reacted with both Southern-transferred rat and mouse P. carinii DNA but not with ferret P. carinii DNA. Thus, P. carinii from rat, mouse, and ferret are genetically diverse. The results are consistent with recently reported antigenic and nucleic acid sequence differences among P. carinii isolates recovered from different hosts.

Nucleoside modifications affect the structure and stability of the anticodon of tRNA(Lys,3).

NMR spectroscopy was used to determine the solution structures of RNA oligonucleotides comprising the anticodon domain of tRNA(Lys,3). The structural effects of the pseudouridine modification at position 39 were investigated and are well correlated with changes in thermodynamic parameters. The loop conformation differs from that seen in tRNA(Phe) and provides an explanation of the critical role of modification in this tRNA.

Studies of the pharmacology and pathology of intrathecally administered 4-anilinopiperidine analogues and morphine in the rat and cat.

In rats, intrathecal alfentanil, lofentanil, sufentanil, fentanyl, and morphine produced dose-dependent elevations in the hot-plate and tail-flick latencies and a powerful suppression of the writhing response. The slopes of the monotonic dose-response curves for the five opioids did not differ significantly. In terms of the hot-plate ED50 after intrathecal injection, the order of potency was as follows: lofentanil (210), sufentanil (29), fentanyl (3), morphine (1), and alfentanil (1). Comparable results were observed in the tail flick. The duration of action was proportional to dose. However, at doses that produced an equal magnitude of inhibition, the duration of action was lofentanil greater than morphine greater than sufentanil greater than alfentanil greater than or equal to fentanyl. Systemically administered naloxone (0.03-1 mg/kg, sc) resulted in dose-dependent antagonism of the antinociceptive effect of intrathecal morphine, fentanyl, alfentanil, and sufentanil. In contrast, intrathecal lofentanil was extremely resistant to antagonism by naloxone. In cats, similar dose-dependent blockade of the thermally evoked skin-twitch response was observed after intrathecal morphine, sufentanil, alfentanil, and fentanyl. As in the rat, the slope of the monotonic dose-response curves did not differ. The relative potency and duration of action after equipotent intrathecal doses were similar to those observed in the rodent. These results suggest that sufentanil, alfentanil, and fentanyl exert their analgesic effects in vivo at a spinal cord site that has properties comparable to those of the site acted upon by morphine. Except for catalepsy in rats, no major behavioral dysfunctions were noted at the ED50 dose of any of the drugs administered. No abnormal morphologic effects of acutely or chronically administered alfentanil and sufentanil were seen, aside from an inflammatory reaction secondary to catheter placement.

Age-related decrease in sensitivity to glucagon and dibutyryl cyclic AMP inhibition of fatty acid synthesis in hepatocytes isolated from obese female Zucker rats.

Hepatocytes were isolated from 3 and 5 month old female genetically obese Zucker rats and their lean littermate controls. An age-dependent loss in sensitivity of fatty acid synthesis to inhibition by both glucagon and dibutyryl cyclic AMP was observed with hepatocytes from the obese rats. Hepatocytes from lean animals were much more sensitive to these agents, regardless of age. Low concentrations of glucagon and dibutyryl cyclic AMP actually produced some stimulation of fatty acid synthesis with hepatocytes prepared from the older obese rats. 5-Tetradecyloxy-2-furoic acid, a compound which inhibits fatty acid synthesis, was a very effective inhibitor of fatty acid synthesis by hepatocytes isolated from all rats used in the study. An inhibition of lactate plus pyruvate accumulation and a strong stimulation of glycogenolysis occurred in response to both glucagon and dibutyryl cyclic AMP with hepatocytes from both age groups of lean and obese rats. The results suggest that with aging of the obese female Zucker rat some step of hepatic fatty acid synthesis becomes progressively less sensitive to inhibition by glucagon and dibutyryl cyclic AMP. This may play an important role in maintenance of obesity in these animals.