Dimeric pentapeptide enkephalin: a novel probe of delta opiate receptors

A dimeric pentapeptide enkephalin (DPE2) consisting of two molecules of [D-Ala 2, Leu 5] enkephalin linked at C-terminal leucine with ethylenediamine, (H-Tyr-D-Ala-Gly-Phe-Leu-NH-Ch2)2 is a bivalent ligand for the delta enkephalin receptors of rat brain and neuroblastoma-glioma hybrid (NG108-15) cells. This new enkephalin analog shows dramatically increased affinity in radioligand assays using whole brain membranes when delta but not mu specific radioligands are employed. When membranes from NG108-15 cells are used, the dimer shows greatly increased activity irrespective of the mu or delta specificity of the tracer. The dimer DPE2 shows a four-fold, "sodium shift" in its IC50 for competition with [3H]naloxone, suggestive of agonist behavior. Agonist activity was confirmed by demonstrating that DPE2 inhibits cyclic AMP production in prostaglandin E1 stimulated NG108-15 cells, and by demonstrating very high potency in the mouse vas deferens bioassay. DPE2 binds to the same delta sites as the delta-selective monomer [D-Ala2, D-Leu5] enkephalin, since the two ligands show complete crossdisplacement. Radiolabeled 3H-DPE2 shows a five-fold higher affinity constant, a 2.5-fold higher association rate constant, and a two-fold lower dissociation rate than the monomer. These results are consistent with the hypothesis that the dimeric pentapeptide enkephalin can bridge two delta receptors. This enkephalin dimer provides a valuable new probe of opiate receptors and their organization in cell membranes.

Autosomal hypophosphataemic bone disease responds to 1,25-(OH)2D3

We diagnosed non X-linked hypophosphataemic bone disease in a 38-month-old girl. Findings included: genu varum, shortened stature, fasting hypophosphataemia (2.3-2.5 mg/100 ml; 0.74-0.81 mmol/l), diminished theoretical renal threshold for phosphate (TmP/GFR), and osteomalacia without rickets. One patient (the father) had fasting hypophosphataemia (2.3-2.7 mg/100 ml; 0.74-0.87 mmol/l) and low TmP/GFR without osteomalacia or shortened stature. Treatment of the girl with 1,25-(OH)2D3 (1 microgram a day) raised the level of serum phosphorus, improved tubular reabsorption of phosphate, and healed the bone deformity; this combination of responses is not present in X-linked hypophosphataemia. There was no correction of hypophosphataemia or TmP/GFR with 1,25-(OH)2D3 treatment (1-3 micrograms a day) in the father.

X-linked hypophosphatemia: effect of calcitriol on renal handling of phosphate, serum phosphate, and bone mineralization

Eleven patients with the Mendelian phenotype of x-linked hypophosphatemia (XLH) were treated with calcitriol [1,25-(OH)2D3] and phosphate. Ten patients had received prior treatment with ergocalciferol and phosphate. Five subjects were prepubertal and six were postpubertal. Response to calcitriol was measured under nonfasting and overnight fasting protocols. Bone biopsies were obtained before and after treatment. Calcitriol (mean dose, 30 ng/kg. day) 1) raised serum phosphorus uniformly in prepubertal patients but in only two of six postpubertal subjects; 2) did not change the theoretical renal phosphate threshold in the total patient group and thus had no effect on the primary transport defect in XLH; 3) improved trabecular bone mineralization in the total patient group, as determined by bone histomorphometry. The beneficial effect on extracellular phosphorus homeostasis was attributed to improved intestinal absorption of phosphorus; improvement in bone mineralization may reflect an additional effect of 1,25-(OH)2D3 on bone itself in XLH. Mild transient hypercalcemia occurred during 0.6% of 3545 treatment days and was readily controlled by adjusting the dosage of 1,25-(OH)2D3.