Calcium channel binding in nerves and muscle of canine small intestine

Characterizing voltage-dependent Ca(2+) channels coupled to VIP release and NO synthesis in enteric synaptosomes

In enteric synaptosomes of the rat, the role of voltage-dependent Ca(2+) channels in K(+)-induced VIP release and nitric oxide (NO) synthesis was investigated. Basal VIP release was 39 +/- 4 pg/mg, and cofactor-substituted NO synthase activity was 7.0 +/- 0.8 fmol. mg(-1). min(-1). K(+) depolarization (65 mM) stimulated VIP release Ca(2+) dependently (basal, 100%; K(+), 172.2 +/- 16.2%; P < 0.05, n = 5). K(+)-stimulated VIP release was reduced by blockers of the P-type (omega-agatoxin-IVA, 3 x 10(-8) M) and N-type (omega-conotoxin-GVIA, 10(-6) M) Ca(2+) channels by ~50 and 25%, respectively, but not by blockers of the L-type (isradipine, 10(-8) M), Q-type (omega-conotoxin-MVIIC, 10(-6) M), or T-type (Ni(2+), 10(-6) M) Ca(2+) channels. In contrast, NO synthesis was suppressed by omega-agatoxin-IVA, omega-conotoxin-GVIA, and isradipine by ~79, 70, and 70%, respectively, whereas Ni(2+) and omega-conotoxin-MVIIC had no effect. These findings are suggestive of a coupling of depolarization-induced VIP release primarily to the P- and N-type Ca(2+) channels, whereas NO synthesis is presumably dependent on Ca(2+) influx not only via the P- and N- but also via the L-type Ca(2+) channel. In contrast, none of the Ca(2+) channel blockers affected VIP release evoked by exogenous NO, suggesting that NO induces VIP secretion by a different mechanism, presumably involving intracellular Ca(2+) stores.

Caveolae from canine airway smooth muscle contain the necessary components for a role in Ca(2+) handling

To explain that bronchial smooth muscle undergoes sustained agonist-induced contractions in a Ca(2+)-free medium, we hypothesized that caveolae in the plasma membrane (PM) contain protected Ca(2+). We isolated caveolae from canine tracheal smooth muscle by detergent treatment of PM-derived microsomes. Detergent-resistant membranes were enriched in caveolin-1, a specific marker for caveolae as well as for L-type Ca(2+) channels and Ca(2+) binding proteins (calsequestrin and calreticulin) as determined by Western blotting. Also, the PM Ca(2+) pump was present but not connexin 43 (a noncaveolae PM protein), the sarcoplasmic reticulum (SR) Ca(2+) pump, or the type 1 inositol 1,4, 5-trisphosphate receptor, supporting the idea that SR-derived membranes were not present. Antibodies to caveolin coimmunoprecipitated caveolin with calsequestrin or calreticulin. Thus some of the cellular calsequestrin and calreticulin associated with caveolin on the cytoplasmic face of each caveola. Immunohistochemistry of tracheal smooth muscle crysosections confirmed the localization of caveolin and the PM Ca(2+) pump to the cell periphery, whereas the SR Ca(2+) pump was located deeper in the cell. The presence of L-type Ca(2+) channels, the PM Ca(2+) pump, and the Ca(2+) bindng proteins calsequestrin and calreticulin in caveolin-enriched membranes supports caveola involvement in airway smooth muscle Ca(2+) handling.

Locations and molecular forms of PACAP and sites and characteristics of PACAP receptors in canine ileum

In canine ileum we investigated the distribution of pituitary adenylate cyclase-activating peptide (PACAP), using immunofluorescence and radioimmunoassay and the binding of 125I-PACAP-27 to membranes. Nerve profiles immunoreactive to PACAP-27, and often to vasoactive intestinal polypeptide (VIP) as well, were found in all plexi, but PACAP was present in approximately 100-fold lesser amounts than VIP. High-performance liquid chromatography analysis of deep muscular plexus (DMP) synaptosomes suggested the presence of PACAP-38, PACAP-27, and a third unidentified molecular form. High- and low-affinity 125I-PACAP-27 binding sites were found in DMP synaptosomes and circular smooth muscle (CM) plasma membranes. In competition studies with DMP membranes, high (H)- and low (L)-affinity dissociation constants (Kd) and maximal binding capacities (Bmax) were as follows: KdH = 66.9 pM, BmaxH = 101 fmol/mg; KdL = 2.18 nM, BmaxL = 580 fmol/mg protein. The binding of 125I-PACAP-27 was fast. Dissociation was slow and incomplete in the presence of unlabeled PACAP-27 but accelerated by pretreatment with guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S). GTP gamma S or cholera toxin treatment eliminated high-affinity binding in both membranes. VIP had approximately 100-fold lower affinity than PACAP-27 in both membranes. Cross-linking studies identified an approximately 70-kDa PACAP receptor in each membrane. Thus PACAP coexists with VIP in ileal enteric nerves and acts on PACAP-preferring, possibly Gs-coupled, receptors in DMP synaptosomes and CM membranes.

Characterization of neurokinin type 1 receptor in canine small intestinal muscle

The binding of [3H]substance P (SP) was localized and characterized in canine small intestine. The highest density of [3H]SP binding occurred in the fraction enriched in circular muscle membranes (Bmax 148 +/- 14 fmol/mg; Kd 0.81 +/- 0.01 nM). In the kinetics studies binding was reversible, yielding a similar Kd value. NK1 agonists and antagonists fully displaced, but ligands at the NK2 and NK3 receptor only partially displaced, [3H]SP binding. The potency order of displacement resembled that for the human NK1 receptor ortholog rather than the rat NK1 receptor. Densities of [3H]SP binding were lower in myenteric plexus and longitudinal muscle membrane fractions, but suggested binding in both neural and muscle membranes. The conclusion is that NK1 receptors occur primarily in canine intestine circular muscle plasma membrane.

Characterization of cholecystokinin receptors in canine intestinal circular muscle

We localized and characterized the binding of [3H](+/-)-L364,718 in canine small intestine circular muscle. The highest densities of [3H]L364,718 binding were located in the fraction enriched in deep muscular plexus synaptosomal membranes. In this fraction [3H]L364,718 binding was of high density (Bmax 136.78 +/- 53.66 fmol/mg) and high affinity (Kd 1.67 +/- 0.74 nM). Kinetics studies revealed that binding was reversible and yielded a similar Kd value. L364,718, CCK-8-S, and L365,260 fully displaced [3H]L364,718 binding, but ligands at CCKB receptors, gastrin-17, and YM022 did not. Therefore, CCKA receptors in canine intestine circular muscle are located on nerve endings.

VIP receptors on canine submucosal synaptosomes

The present study characterized [125I]VIP binding to synaptosomes from the submucosa of canine small intestine. Studies of saturation, competition binding, and kinetic studies revealed high- and low-affinity binding sites. Studies with GTP-gamma-S and cholera toxin suggested that the receptor was coupled to a G-protein, possibly Gs. Competition with VIP analogs suggested that the N-terminal end of the molecule played the major role in determining affinity and that this receptor was for VIP, not PACAP. Cross-linking VIP to the receptor revealed a single peptide (M(r) congruent to 60,000). We suggest that VIP may act to modulate mediator release from enteric nerve endings.

Calcium-channel blockers and gastrointestinal motility: basic and clinical aspects

Several calcium-channel blockers currently in use for the treatment of cardiovascular disorders have recently been tested for their effects on gastrointestinal motility. The rationale for this approach centers on the concept that calcium-channel blockers are at least as potent in inhibiting intestinal smooth muscle as in relaxing vascular smooth muscle. This review will give an outline of the most recent findings on the role of calcium and calcium channels in smooth muscle and neuronal function in the digestive system. It will also consider the mechanisms by which calcium-channel blockers may affect gastrointestinal motility and assess potential clinical applications in gastroenterology. The main goal for researchers in this field will be the development of gut-selective agents, with no cardiovascular side effects.

Inhibition of endogenous acetylcholine release by blockade of voltage-dependent calcium channels in enteric neurons of the guinea-pig colon

The effects on acetylcholine release from the guinea-pig colon of the N-type calcium channel blocker omega-conotoxin GVIA (omega-conotoxin), the L-type calcium channel blocker nifedipine and the putative blocker of T-type channels, flunarizine, have been investigated. Endogenous basal acetylcholine release and electrically (1 Hz, 1 ms, 450 mA)-evoked overflow in the presence of cholinesterase inhibitor were studied. omega-Conotoxin (1-10 nM) and nifedipine (0.03-3 microM) dose-dependently inhibited basal and electrically-evoked acetylcholine release. Maximal inhibition of basal or electrically-evoked acetylcholine release was about 40% for nifedipine and about 75% for omega-conotoxin. The potency of nifedipine was inversely related to the external calcium concentration: its EC50 value in low-calcium medium (0.5 mM) was as low as 12 nM. Flunarizine inhibited acetylcholine release only at concentrations higher than 0.2 microM. Our results are consistent with an involvement of N- and L-type calcium channels in the control of the endogenous acetylcholine release from the guinea-pig colon.