Heterogeneity of neurokinin1 binding sites in porcine respiratory tract

Biologic activity of the tachykinins on lamb and sheep gallbladder

In this study, we examined the activity of the tachykinins (TKs) on lamb and sheep isolated gallbladder and whether the TKs are involved in the capsaicin-induced activity in these tissues. Substance P (SP) and physalaemin (PHYS) contracted lamb gallbladder, PHYS-induced striking tachyphylaxis. This tissue was nearly insensitive to neurokinin A (NKA), neurokinin B (NKB), septide, and capsaicin. As in lamb tissues, SP and PHYS both contracted sheep gallbladder although PHYS induced no tachyphylaxis. At doses that had no effect on lamb tissue, NKA, NKB, septide, and capsaicin contracted sheep gallbladder. Our findings indicate that TK receptors differ in adult and young ovine gallbladder. The activity of PHYS on lamb gallbladder could depend on the existence of an unusual binding site, carrying one or more residues critical for the N-terminal sequence present in PHYS but not in SP.

Ontogeny of neurokinin-1 receptors in the porcine respiratory system

We characterized the ontogeny of NK-1 receptor agonist affinity (Kd) and density (Bmax) in membranes from tracheal epithelium, smooth muscle, and lung of pigs aged 1-7 days, 8-21 days, and adult in comparison to contractile responses in vitro. Affinity of [125I] Bolton-Hunter substance P ([125I]BH-SP) in epithelium and smooth muscle was three- to fourfold lower in young piglets than in adults. The Bmax of NK-1 sites in epithelium was elevated by more than twofold at 8-21 days relative to 1-7 days piglets and adults. In the lung, NK-1 density as well as affinity was lower than in trachea, regardless of age. In all three groups, [125I]BH-SP binding was potently inhibited by Gpp(NH)p, in both trachea and lung, implying coupling to G-proteins. Inhibition by Gpp(NH)p was most potent in the adult relative to younger animals, in both tracheal epithelium and smooth muscle. Functional sensitivity to the NK-1 agonists substance P and septide was reduced in neonates, as shown by the higher concentration of agonist required to elicit contractile responses. We conclude that the reduced sensitivity of newborn piglet airways to substance P reflects immaturity of G-protein coupling to NK-1, independent of receptor density.

Muscarinic receptor binding sites of the M4 subtype in porcine lung parenchyma

Muscarinic acetylcholine receptors regulate distal airway resistance and secretion. The subtype expressed in the lung in different species remains uncertain. It has recently become possible to identify the M4 subtype by careful comparison of antagonist affinities. We characterized the binding of [3H]quinuclidinyl benzilate ([3H]QNB) to muscarinic receptors in cell membranes from lung parenchyma of 2-8 week old pigs in comparison to cloned human M3 and M4 receptors expressed in COS cells, to M2 in rat atria and to M4 in bovine adrenal medulla. In porcine lung, [3H]QNB bound with high affinity (Kd = 95 +/- 9 pM) to a single homogeneous population of muscarinic receptor sites (Bmax = 340 +/- 10 fmol/mg protein). Competition studies showed that the affinity (expressed as pKi) of 3 selective blockers was in close agreement between pig lung and cloned human m4 (r = 0.996). A series of 10 blockers showed affinities closely matching reported values for M4 receptors of the adrenal medulla (r = 0.965). Conversely, affinity values in porcine lung differed significantly (P < 0.05, t-test) from those determined in parallel with either human cloned M3 or with rat atria expressing the M2 subtype. We conclude that pig lung muscarinic receptor binding sites most closely resemble the M4 subtype, in contrast to the M3 subtype typical of large airways in this species.

Membrane localization and guanine nucleotide sensitivity of medullary I1-imidazoline binding sites

Imidazoline binding sites are labeled by [3H]clonidine (I1) or by [3H]idazoxan (I2). I2-sites are mitochondrial. The subcellular localization of I1-sites in brain is unknown. Crude membranes from bovine rostral ventrolateral medulla (RVLM) were further purified by discontinuous sucrose density gradient. Fractions were assayed for I1-site density (Bmax) with [125I]p-iodoclonidine. Nonspecific binding was defined by 10 microM BDF-6143, and alpha 2-adrenergic binding was defined by 10 microM epinephrine. The proportions of I1 and alpha 2 in mitochondrial fractions were similar (28 +/- 3 and 24 +/- 4%, respectively), and both I1 and alpha 2 showed the greatest enrichment within the membrane-enriched fraction (58 +/- 13 and 38 +/- 4%). The myelin fraction contained a higher proportion of alpha 2 than I1 (38 +/- 4 and 15 +/- 2%), consistent with expression of alpha 2, but not I1, by glia. The enrichment of I1 and alpha 2 in cellular membranes and alpha 2 in myelin was confirmed by further purification of these fractions over a second discontinuous gradient. Following irreversible inactivation of alpha 2, the remaining I1 sites in RVLM crude membranes were inhibited by Gpp(NH)p but not by ATP. We conclude that I1-imidazoline sites are non-mitochondrial membrane proteins sensitive to guanine nucleotide and may be functional receptors.