[3H]L-657,743 (MK-912): a new, high affinity, selective radioligand for brain alpha 2-adrenoceptors.

L-657,743 (MK-912), a highly potent and selective alpha 2-adrenoceptor antagonist was tritiated to a high specific activity and its binding characteristics to brain tissue were determined. The specific binding of [3H]L-657,743 to rat cerebrocortex was saturable, reversible, and dependent on tissue concentration. In saturation studies, [3H]L-657,743 binding was resolved into two high affinity components exhibiting Kd values of 86 pM and 830 pM with densities of 82 fmol/mg protein and 660 fmol/mg protein, respectively. Based on the binding potencies of a variety of compounds with differing receptor selectivities, the sites labeled by [3H]L-657,743 were characteristic of alpha 2-adrenoceptors. In contrast to alpha 2-antagonists, alpha 2-agonists displayed shallow competition curves. In the presence of 100 microM GTP, Gpp(NH)p or 150 mM NaCl, the competition curve for epinephrine was shifted to the right, whereas that for yohimbine was unaffected. In studies utilizing human cerebrocortical tissue, [3H]L-657,743 also bound with high affinity to sites characteristic of alpha 2-adrenoceptors.

Multiple mechanisms for inhibition of excitatory amino acid receptors coupled to phosphoinositide hydrolysis.

Excitatory amino acid (EAA) analogues activate receptors that are coupled to the increased hydrolysis of phosphoinositides (PIs). In these studies, hippocampal slices were prepared from neonatal rats (6-11 days old) to characterize the effects of EAA analogues on these receptors. The concentrations of ibotenate and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) required to evoke half-maximal stimulation (EC50 values) were 28 and 51 microM, respectively. Although the data for stimulation of PI hydrolysis by ibotenate and trans-ACPD were best fit to theoretical curves that had Hill slopes of 1, data for stimulation of PI hydrolysis by quisqualate were best fit to two sites. The EC50 values were 0.43 and 44 microM. The high-affinity sites were 70% of the total. A number of EAA analogues were tested for inhibition of PI metabolism. One of these, L-aspartate-beta-hydroxamate (L-A beta HA), was identified as a novel inhibitor of this response. L-A beta HA was equipotent as an inhibitor of PI metabolism stimulated by ibotenate, quisqualate, and trans-ACPD. The data for this inhibition were best fit to two sites. Between 32 and 48% of the total sites had high affinity with IC50 values in the range of 1.2-6.3 microM. The low-affinity sites had IC50 values between 610 and 2,700 microM. DL-2-Amino-3-phosphonopropionate (DL-AP3) was also equipotent as an inhibitor of PI hydrolysis stimulated by ibotenate, quisqualate, and trans-ACPD (IC50 values were 480-850 microM). In contrast to the data for L-A beta HA, the data for DL-AP3 were best fit to a single site. Both of these inhibitors reduced the maximal response caused by the agonists, consistent with noncompetitive mechanisms of action. Several experiments were designed to examine potential mechanisms for these noncompetitive effects. These studies suggest that either L-A beta HA and DL-AP3 bind to a site on the receptor and irreversibly block activation of the receptor, or that these inhibitors act via a distinct site that specifically regulates EAA receptors coupled to PI hydrolysis.

Myositis resulting from disseminated cryptococcosis in a patient with hepatitis C cirrhosis.

We report a case of myositis that resulted from disseminated cryptococcosis in a patient with hepatitis C cirrhosis. One year after cessation of treatment, the patient remains symptom free with negative results of serum cryptococcal antigen tests and negative culture results.