Poul Staun-Olsen

The regional distribution of receptors for vasoactive intestinal polypeptide (VIP) in the rat central nervous system

The regional distribution of receptors for vasoactive intestinal polypeptide (VIP) was studied in the rat central nervous system (CNS). The specific binding was highest in cerebral cortex, limbic forebrain and cerebellum, whereas moderate to low binding was found in hypothalamus, thalamus, brainstem and pituitary. The lowest binding was observed in pons and spinal cord. Scatchard analysis showed curvilinear plots with upward concavity, which was interpreted as two classes of binding sites. The Kd values were similar in all regions and calculated as 2.4 and 62 nmol/liter, respectively. The variations of specific [125I]VIP binding were due to differences in the total amount of receptors and were in the range of 1.7-8.6 pmol per mg protein. The regional distribution of VIP receptors was parallel with the occurrence of VIP-containing nerve terminals with exceptions of cerebellum, olfactory areas and nucleus caudatus, where a greater number of receptors than expected from the VIP content was found. In these regions, VIP may interact with receptors for a different, but homologous neuropeptide. In conclusion, the regional distribution of VIP receptors in CNS gives further evidence for the role of VIP as a central neurotransmitter.

Insulin receptors in rat brain cortex. Kinetic evidence for a receptor subtype in the central nervous system.

Binding kinetics of porcine 125I-insulin were studied in synaptosomal and microsomal fractions of rat brain cortex. Receptor binding was temperature- and pH-dependent with optimum at 4 degrees C and pH 8.0-8.3. At 15 degrees C, steady state binding was heterogenous, and Scatchard analysis revealed two classes of receptors with Kd of 2 nmol/l and 40 nmol/l in amounts of 50 pmol/g and 200 pmol/g of membrane protein. Dissociation kinetics were biexponential with T1/2 of about 5 min and 180 min, and in contrast to other cell-types, not influenced by negative cooperativity. No receptor-mediated insulin degradation was detectable at 37 degrees C in the presence of bacitracin. Insulin analogues inhibited 125I-insulin binding with potencies relative to porcine insulin (%): human insulin 100, rat insulin (I + II) 71, coypu insulin 47, rat multiplication stimulating activity 8, porcine proinsulin 5, among which the three last values were significantly higher than in rat liver and fat cells. No competition was observed with porcine relaxin and mouse nerve growth factor up to about 1 mumol/l. Receptors were present in all regions of central nervous system with highest concentrations in the cerebral cortex, cerebellum and olfactory bulb, and lowest in the pons, medulla oblongata and spinal cord. In conclusion, insulin receptors in rat brain cortex are functionally different from other tissues regarding the insulin specificity and the absence of negative cooperativity. It is suggested that an insulin receptor subtype in rat brain mediates the growth activity of insulin on nerve cells.

Influence of pregnancy and sex steroids on concentration, motor effect and receptor binding of VIP in the rabbit female genital tract

The influence of sex steroid and pregnancy on the tissue concentration, uterine motor effect and receptor binding of VIP has been studied in the female genital tract of pregnant rabbits and oophorectomized rabbits during progesterone and/or oestrogen substitution. 1. (1) The concentration of immunoreactive VIP was high in the vagina and cervix, and lower in the uterine body of both pregnant and non-pregnant rabbits. A significant decrease in the VIP concentration (pmol/g wet weight) of the uterine body was observed toward term of pregnancy. The total uterine content of VIP, however, seems unchanged. Treatment of oophorectomized rabbits with ovarian steroids had no effect on the VIP concentration. 2. (2) The sensitivity for and potency of VIP on the relaxation of uterine muscle was significantly higher in oophorectomized rabbits treated with a combination of progesterone and oestrogen than in control rabbits. No difference was observed between non-pregnant and pregnant rabbits. 3. (3) The degradation and binding affinity for 125I-labelled VIP was highest in oophorectomized rabbits substituted with both oestrogen and progesterone. In the pregnant rabbits, the amount of receptors was decreased near term. In conclusion, sex steroids are able to influence the motor effect of VIP at the receptor level, but have no effect on the VIP concentration in the female genital tract.

Multiplicity of receptors for vasoactive intestinal polypeptide (VIP): Differential effects of apamin on binding in brain, uterus and liver

Apamin is a neurotoxic octadecapeptide from bee venom, which has been shown to inhibit the non-adrenergic, non-cholinergic inhibitory innervation of the smooth muscle of the gut. Since vasoactive intestinal polypeptide (VIP) has been proposed as a possible inhibitory neurotransmitter, the effect of apamin on the receptor binding of 125I-VIP was studied using the following assays: (1) isolated synaptosomes from rat cerebral cortex, (2) crude plasma membranes from hog uterine smooth muscle, and (3) purified plasma membranes and isolated hepatocytes from hog liver. Apamin inhibited the receptor-bound 125I-VIP on membranes from brain or myometrium, although the binding affinity was 100-1000 times lower than for VIP. The displacement curves for VIP and apamin were parallel suggesting that apamin interacts with both the low and high affinity VIP receptors. In membranes and cells from liver, apamin was unable to displace receptor-bound 125I-VIP in concentrations up to 50 mumol/l. The findings suggest that the VIP receptors in liver are different from those in the brain cortex and myometrium.

VIP binding sites on synaptosomes from rat cerebral cortex: Structure-binding relationship

The structural requirements for VIP interaction with receptors on synaptosomes from rat cerebral cortex was investigated by the ability of VIP and VIP fragments, secretin analogues and fragments, peptides of the VIP/secretin family and several other regulatory peptides to inhibit specific 125I-VIP binding. Only large VIP fragments interacted with the VIP receptors with potencies relative to VIP ranging from 0.9-0.006%. The rank order of inhibition was: VIP 7-27 greater than VIP 11-28 greater than VIP 1-22-NH2 greater than VIP 16-28. Shorter fragments: VIP 18-28; VIP 18-28-NH2; VIP 19-28; VIP 21-28; VIP 22-28; VIP 1-18; VIP 1-18-NH2; VIP 1-10-NH2; VIP 1-6; VIP 16-20 and VIP 16-19 had no effect. Secretin fragments and analogues inhibited 125I-VIP binding with potencies of 2.2-0.01% relative to VIP in the order; secretin greater than (Ala4, Val5) secretin greater than (D-Ala4) secretin greater than (D-Phe6) secretin greater than secretin 5-27 greater than secretin 14-27. Other peptides of the VIP/secretin family inhibited 125I-VIP binding with potencies of 200-1%; avian VIP greater than porcine VIP greater than PHI = secretin greater than human GRF, whereas glucagon and GIP showed no inhibition. Among twenty-five other regulatory peptides only avian PP and somatostatin were inhibitors with relative potencies of 0.02% and 0.03%, respectively. In conclusion it may be emphasized that the intact VIP molecule is essential for VIP interaction with its receptors in the rat brain cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoactive Intestinal Polypeptide (VIP): Specific Receptors on Smooth Muscle Membranes from Porcine Uterus

Abstract Receptors for vasoactive intestinal polypeptide (VIP) studied in crude smooth muscle membranes (20,000 x g pellet) from porcine uterus. The receptor binding of 125I-VIP was dependent on time, temperature, membrane protein concentration and pH. The binding process was saturable, reversible and specific. At steady state the binding of 125I-VIP was displaced by unlabeled VIP and the concentration with half-maximum inhibition was 3 nmol/l. The Scatchard plot was linear suggesting one class of non-cooperative receptors. The presence of specific receptors for VIP in myometrial membranes supports the hypothesis that VIP plays a physiological role as neurotransmitter in control of uterine smooth muscle activity.

Multiple Functions of Peptides: Insulin and Vasoactive Intestinal Polypeptide Bind to Receptors in Liver and Brain

Abstract Receptors for insulin and VIP have been studied in isolated pig hepatocytes and purified synaptosomes from rat cerebral cortex. The binding was characterized by high affinity, specificity and partial reversibility. A comparison between receptors in liver and brain for each of the two peptides revealed differences in binding characteristics. The presence of receptors in liver and brain suggests a physiological role of insulin and VIP in both tissues. The differences in binding characteristics may discriminate between their biological functions. In liver the two peptides have a function as hormones, whereas in brain insulin and VIP are neurotransmitters.

The Complex Kinetics of Insulin Binding to Receptors on Isolated Pig Hepatocytes

Abstract The dissociation and association kinetics of 125I-insulin binding to receptors in isolated pig hepatocytes were studied at 37 °C or 15 °C. The dissociation of 125I-insulin was biexponential with a rapid and a very slow component. The rate and size of the 2 components were influenced by preincubation time, temperature and concentration of added insulin. The association of 125I-insulin in the presence of insulin showed decreased initial rate and steady-state binding, when cells were preincubated 90 min with insulin as compared to preincubation without insulin. It is concluded, that the formation of a slowly dissociating complex of insulin and receptor during association results in “down regulation” of insulin-receptor binding.