Computer analysis of the effect of beta-endorphin and dynorphin and related compounds on opioid binding to mouse brain membrane.

The binding of three tritiated opioid agonists--dihydromorphine, D-ala2-D-leu5-enkephalin and ethylketocyclazocine--was subjected to competition by unlabeled beta-endorphin, dynorphin-(1-13), and various fragments of these peptides, and the results analyzed by a computer program that we developed in an earlier study [11]. Peptides in both groups bound with highest affinity to sites 1 and 3 in our 4-site model, corresponding to the mu and delta sites of conventional classifications, with the dynorphin peptides also interacting with site 2, the kappa site. These results are discussed in relationship to the possible biological roles of these peptides as analgesics or as modulators of analgesia.

Effects of tolbutamide pretreatment on the rate of conversion of newly synthesized proinsulin to insulin and the compartmental characteristics of insulin storage in isolated rat islets.

Tolbutamide (1 g/kg body wt) was administered to male rats for 3 days to determine the effects of this pretreatment on subsequent insulin biosynthesis and compartmental storage characteristics of freshly isolated islets. Islets were isolated 16 h after the last tolbutamide administration, at a time when fed plasma glucose concentrations were normal. Islet glucagon was unchanged but insulin content was significantly reduced (38 +/- 1.2 ng IRI/islet from seven untreated rats versus 7.9 +/- 1.2 ng IRI/islet from eight treated rats). After tolbutamide pretreatment, the rate of incorporation of 3H-leucine into islet proinsulin was unchanged, but the t1/2 of labeled proinsulin-to-insulin conversion was significantly (P less than 0.001) decreased from 36 to 20 min. After treatment, actual rates of glucose-stimulated insulin secretion were 50% lower, however, because due to the proportionately greater depletion of islet insulin content, the fractional rate of secretion was increased two-fold. After treatment, there was evidence of compartmental, heterogeneous insulin storage, and glucose still marked newly synthesized insulin for preferential release; however, the differential release of new and old insulin converged rapidly with time. Mathematical integration of the data suggested dilution of the newly synthesized insulin compartment with unlabeled insulin during the chase period, but additionally indicated more rapid mixing of newly synthesized with previously stored, unlabeled insulin. Thus, tolbutamide-treated rats partially compensated for acute insulin depletion by increasing the rate of proinsulin-to-insulin conversion, but not increasing the rate of proinsulin biosynthesis; doubling the glucose-stimulated fractional secretory rate of the depleted cellular insulin storage compartment; and retaining compartmental storage characteristics but mixing newly synthesized insulin more rapidly with the compartment of previously stored, unlabeled insulin.

Pharmacokinetic approaches to drug distribution in the cerebrospinal fluid based on ventricular administration in beagle dogs.

Two mathematical approaches are described to approximate the distribution of compounds (e.g., drugs) in the cerebrospinal fluid (CSF) downstream of or distal to both the ventricular injection site and the cisterna magna sampling site. The first approach uses a graphic representation and is, in essence, model independent; the second approach considers the geometry and physiology of CSF distribution and clearance. In all studies, radiolabeled inulin was used as an "internal standard" since it is not metabolized and is eliminated from the CSF primarily by bulk flow. Temporal comparison of the study compound to radiolabeled inulin in the cisternal CSF allowed testing of these models in beagle dogs. One use of this data is in the estimation of the drug exposure integral for antineoplastic drugs administered in the CSF to treat leptomeningeal neoplasia.

Relationship of octanol/water partition coefficient and molecular weight to cellular permeability and partitioning in s49 lymphoma cells.

We have used modified standard methods and derived new formulae to quantitate cell permeability (P), cell/media partitioning (λ), and intracellular sequestration or binding rate constants (m) for cultured S49 murine lymphoma cells in suspension. Using 15 standard compounds and anticancer drugs, we found quantitative relationships among log P, log PO (octanol/pH 7.4 buffer partition coefficient), and molecular weight (MW) such that logP = -4.5 + 0.56log (PO(MW)(-1/2)). A good correlation among P, λ, and MW was also determined with λ = 0.67 + 5890 gm(1/2) cm(-1) sec (P (MW)(1/2)). These studies show that there is a strong partitioning (λ) dependence to molecular weight and permeability that can be predicted even for known carrier-transported and biotransformable compounds. Furthermore, results of this study show that the slope of the plot of permeability and lipophilicity is not necessarily unity as has been postulated from the results of other studies.

Heterogeneity and compartmental properties of insulin storage and secretion in rat islets.

To investigate compartmental properties of insulin storage and secretion, isolated rat islets were used for pulse-labeling experiments, after which proinsulin and insulin were purified rigorously. Processing of proinsulin to insulin neared completion by 3 h without additional loss of either radioactive peptide by cellular or extracellular proteolysis. The amount of labeled hormone rapidly diminished in islets; it was secreted at a higher fractional rate than immunoreactive insulin, resulting in secreted insulin's having a higher specific activity than the average cellular insulin. Newly synthesized insulin, therefore, was secreted preferentially. Changes in the specific activity of secreted and cellular insulin with time were consistent with changes predicted for islets containing 33% of their total insulin in a glucose-labile compartment. Predictions were based on steady-state analysis of a simple storage-limited representation of B cell function. Islets from either the dorsal or ventral part of the pancreas also contained 33% of their total insulin in a glucose-labile compartment. The same compartment was mobilized by 20 mM glucose, 50 mM potassium + 2 mM glucose, or 20 MM glucose + 1 mM 3-isobutylmethylxanthine as indicated by the specific activity ratio of secreted vs. cellular insulin, even though average secretion rates with these stimuli differed by more than threefold. In the absence of calcium, the effectiveness of 20 mM glucose as a secretagogue declined markedly, and the older stored insulin was preferentially mobilized because secreted insulin had a lower rather than a higher specific activity than cellular insulin. Results provide insight into the mechanisms of nonrandom mobilization and secretion of insulin form the B cell.

Heuristic modeling of drug delivery to malignant brain tumors.

It is apparent that chemotherapy against malignant brain tumors is generally ineffective. While some agents are more effective than others, none appreciably alters the clinical course of and the poor prognosis for patients with brain tumors. Even though new and more effective agents are being or will be developed, chemotherapy depends as much on the delivery of drug as it does on the drug used. Therefore, we have defined factors that we believe are of primary importance in drug delivery to brain tumors, and, using computer simulation, we have modeled the effects of these factors. In this article we discuss (a) the extent of the "breakdown" in the blood-brain barrier (BBB) that accompanies the development of malignant tumors in the brain, (b) factors that influence drug transport from tumor capillaries to tumor cells at varying distances from the capillaries, (c) the problems inherent in drug delivery from a well-vascularized tumor outward to normal brain tissue that might harbor malignant cells but that does not have leaky vessels (i.e., normal BBB), and (d) the difficulties in drug delivery from a well-perfused, highly permeable outer tumor shell to a central, poorly perfused tumor core.

In situ drug delivery.

Drug delivery to tumours and the accessibility of tumour cells to drugs remains a problem of foremost importance and is affected by many factors. We will discuss models we have developed for drug delivery from tumour capillaries to tumour cells, and drug delivery from well perfused, well oxygenated tumour regions to less well perfused hypoxic regions in terms of computer simulation of drug delivery for the anticancer agent BCNU and the hypoxic cell radiosensitizer misonidazole.

Role of rate of change of glucose concentration as a signal for insulin release.

In the isolated perfused pancreas, the amount of insulin secreted in response to a glucose stimulus was evaluated as a function of the maximum rate of change of the stimulus. Range of glucose concentration, total amount of glucose, total duration of the changing stimulus, the average concentration, and the average rate of change of concentration were the same. Fast changes stimulated more insulin secretion than slow changes, indicating that the rate-sensor property was inherent in the control of insulin release.

Permeability characteristics of brain adjacent to tumors in rats.

The brain immediately surrounding 9L sarcoma and Walker 256 carcinosarcoma was evaluated, using radioactive water, albumin, red blood cells, urea, and sodium to quantitate isotopic exchange and permeability in the brain adjacent to tumor (BAT), normal brain, and, to a lesser extent, tumor. Exchange between blood and BAT for 14C-urea and 22Na averaged 53% of that for comparable regions of normal brain. This reduction in exchange is not explainable by differences in capillary surface area for transcapillary exchange in the BAT. This reduction in capillary permeability in the BAT could be detrimental to the delivery of watersoluble and rapidly binding drugs.