K.W. Chiu

Calcitonin gene-related peptide in the bullfrog, Rana catesbeiana: localization and vascular actions

Calcitonin gene-related peptide (CGRP) was found using radioimmunoassay in the brain and spinal cord of the bullfrog, Rana catesbeiana. The brain extracts were found to contain 241.7 +/- 68.1 pg/mg tissue, and the spinal cord contained 1753.0 +/- 96.8 pg/mg tissue. An intense immunocytochemical reaction was observed in the dorsal spinal cord. Vascular studies using helical strips of the dorsal aorta, iliac artery, and femoral artery showed CGRP to exert a vasorelaxant effect which was most pronounced in the femoral artery and minimal in the aorta. As in the rat, CGRP was shown to exert its vasorelaxant effect by inhibiting the mobilization of intracellular calcium.

The vasorelaxant effect of atrial natriuretic peptide in the frog

The vasorelaxant effect of atrial natriuretic peptide (ANP) in two species of frogs, Rana catesbeiana and R. tigrina, were studied in vitro. Isolated arterial strips were prepared from the aortic loop, dorsal aorta, iliac, and femoral arteries in the frog. These were stimulated to contract with KCl, norepinephrine, or arginine vasotocin. When maximal contraction was attained, ANP was added to assess if it could relax the strips. Data show that ANP is vasorelaxant in these preparations. The sensitivity of these tissues depends on the contractile agents, e.g., KCl-stimulated preparations from R. catesbeiana did not respond to ANP. Data also suggest that ANP action elicited in the frog vascular tissue is due to an inhibitory effect on the mobilization of the intracellular calcium store and/or calcium influx from extracellular space to initiate contraction.

Neurohypophysial hormones and cardiac activity in the frog, Rana tigrina, and in the snake, Ptyas mucosa.

The cardiac effects of neurohypophysial hormones (NH) in the frog, Rana tigrina, and in the snake, Ptyas mucosa, were studied in isolated atrial preparations. Arginine vasopressin (AVP), vasotocin (AVT), isotocin (ISN), mesotocin (MSN), and oxytocin (OXY) produced dose-related positive chronotropic and inotropic responses in the frog atria. AVT was the most potent. In the snake, AVP, AVT, MSN, and OXY produced dose-related positive inotropic but not chronotropic changes. OXY was the most potent. Data indicate that there is a direct action of NH on the heart in both animals, the atrial rate and tension responses to NH are not coupled, and judging by the efficacy, AVT and OXY might be the natural NH in the frog and snake, respectively, which may account for the high concentrations required of other NH to be cardiac stimulating.

Blood pressure homeostasis in the snake, Ptyas korros.

Three putative pressor systems, the alpha-adrenergic system (AS), the renin angiotensin system (RAS), and the arginine vasotocin system (AVT-S), were studied for their roles in blood pressure regulation and their possible interactions in the rat snake. Ptyas korros. Norepinephrine (NE), angiotensin I (ANG I), and arginine vasotocin (AVT) increased the mean arterial pressure (MAP) of the snake while administration of phentolamine, an alpha-adrenergic antagonist, and captopril, an ANG-converting enzyme inhibitor, but not KBIV24, an AVT antagonist, decreased the MAP. Treatment with any combination of two of these antagonists/inhibitor invariably decreased the MAP. Treatment with the agonist of the remaining third system invariably returned the MAP to normal or above. Phentolamine and KBIV24 attenuated the vasopressor effect of ANG I. Phentolamine and captopril enhanced the vasopressor effect of AVT. The pressor effect of NE was not altered by KBIV24 and captopril. It was concluded that there were at least two pressor systems (AS and RAS) regulating the basal MAP in the snake. There was also interaction among the three systems which could affect the MAP.

The mechanism of action of angiotensin II in the rat snake, Ptyas korros.

The hindquarter of the rat snake, Ptyas korros, was isolated for perfusion study in order to investigate the mechanism of action of angiotensin (ANG) II. Both ANG II and norepinephrine (NE) produced concentration-dependent increases in perfusion pressure. [Sar1, Ile8] ANG II significantly attenuated the response to ANG II but not that of NE. Phentolamine significantly reduced the responses to both ANG II and NE, with much greater inhibition on NE than ANG II. These findings suggest the presence of ANG II receptors and alpha-adrenergic receptors in the hindquarter. Since tyramine did not exert any increase in perfusion pressure, the release of ANG II-stimulated catecholamines was probably not involved in the increase in perfusion pressure to ANG II in the present preparation. The partial attenuation of the response to ANG II by phentolamine, therefore, suggests that ANG II may act directly on both specific ANG II receptors and alpha-adrenergic receptors.

Cardiac activity of some peptide hormones in the frog, Rana Tigrina

1. The cardiac responses of isolated frog (Rana tigrina) atria to peptide hormones were studied. 2. Calcitonin gene-related peptide (CGRP), arginine vasotocin (AVT), bovine parathyroid hormone fragment (bPTH-(1-34)) and oxytocin (OXY) produced dose-related positive chronotropic and inotropic responses; atrial natriuretic peptide (ANP) was negative chronotropic and inotropic; cholecystokinin (CCK), vasoactive intestinal peptide (VIP) were without effects. 3. The dose-related responses under bPTH-(1-34) stimulation but not CGRP or AVT were attenuated in the presence of ANP (300 ng/ml, approximately 0.98 x 10(-7) M). As expected ANP decreased the basal AR and AT responses of the isolated atria and the inhibitory effects were dose-dependent. 4. As shown previously, propranolol blocked the atrial tension stimulated by bPTH (1-34) but did not alter the cardiac responses to CGRP and AVT. 5. In the presence of beta-adrenergic blocker (propranolol 10(-7) M) or ANP (10(-7) M), the AR and AT changes under ISO stimulation in the frog were also decreased. 6. These cardiac changes suggest the cardiac inhibitory effects of ANP are related to beta-adrenoceptor activity and ANP might be a beta antagonist.

The cardiac effects of parathyroid hormone in the tiger frog, Rana tigrina

1. The cardiac effects of the N-terminal (1-34) peptide fragment of bovine parathyroid hormone [bPTH-(1-34)] on isolated atria were examined in the frog, Rana tigrina. 2. bPTH-(1-34) produced dose-related inotropic response but no chronotropic response. This inotropic response varied at different times of the year. 3. The inotropic effect of bPTH-(1-34) was attenuated in the presence of verapamil and imidazole. 4. The mechanism of action of bPTH-(1-34) is probably a stimulation of calcium influx directly or indirectly via cAMP production.

Structure-cardiovascular activity relationships of parathyroid hormone

The effects of parathyroid hormone (PTH) analogues on isolated rat tail arteries and frog arteries and heart were examined. Synthetic bovine PTH fragment 1 to 34 [bPTH-(1-34)], porcine PTH-(1-34), [Ala1] pPTH-(1-34), and [Nle8] pPTH-(1-34) were tested for vasorelaxant activity on helical strips of the rat tail artery and the frog iliac and femoral artery, preconstricted with arginine vasopressin or arginine vasotocin in the rat and the frog, respectively. For cardiac activity, PTH analogues were administered to isolated frog atria, and atrial rate and atrial tension (AT) were measured. The N-terminal amino acid alanine appears to be pivotal for vasorelaxant activity of PTH molecules on the rat tail artery strips. Data from the frog preparations supported the finding in the rat, although the iliac and femoral arterial preparations differed in their responses. The N-terminal serine in pPTH-(1-34), compared with the other PTH analogues, appeared to be responsible for different cardiac responses, being negatively chronotropic and without effect on AT.

Cardiac activity of calcitonin gene-related peptide in amphibian species (Rana tigrina, Triturus sp).

1. Calcitonin gene-related peptide (CGRP) produced dose-related positive chronotropic and inotropic responses in isolated atria of a newt, Triturus sp. and a frog, Rana tigrina. It was apparent that CGRP was stimulatory on the heart of the amphibian species. 2. In the frog, these dose-related responses were attenuated in the presence of a CGRP antagonist (hCGRP 3 x 10(-8) M). This was indicative of the presence of cardiac CGRP receptors. 3. In the presence of beta-adrenergic (propranolol 10(-6) M) and calcium channel (verapamil 10(-8) M, but not nifedipine 10(-8) M) blocker, the basal AR in the frog were decreased. AR induced by CGRP were increased however. 4. The maximal AR and AT responses of the isolated atria to CGRP were not affected by the presence of propranolol. 5. In the presence of calcium channel blockers (verapamil and nifedipine), the maximal AT response was attenuated. These changes suggest the cardiac effects of CGRP are related to calcium mobilization.