[The vasodilation and its mechanism of C-type natriuretic peptide]

AIM AND METHODS

With routine blood vessel perfusion in vitro, the vasodilation and its mechanism of C-type natriuretic peptide, a new member of natriuretic peptide family were observed in rabbits.

RESULTS

CNP had the dose-dependent vasodilation effects on abdominal artery and celiac vein at the range of 10(-10)-10(-6) mol/L, its action on vein was just like nitroglycerin, its action on artery was weaker than that of ANP. Atropine (10(-7) mol/L), Regitine (20 microg), and indomethacin (20 microg) had nothing effect on the vasodilation of CNP on abdominal artery. But glibenclamide (10(-6) mol/L) and propranolol (10(-6) mol/L) could decrease the vasodilation of CNP obviously, in addition, CNP couldn't inhibit the vasoconstriction of NE.

CONCLUSION

(1) CNP might be kind of venous systemic vasodilator, and it was also a arterial selective peptide controlling the vessel tone. (2) CNP dilated the vessel at least through two ways: (a) ATP-sensitive K+ channel, (b) beta-receptor.

[The blocking effects of extracellular Mn2+ on the inward rectifier potassium channel (IRK1)]

AIM AND METHODS

Two-microelectrode voltage clamp (TEV) method was used to study the blocking effects of extracellular Mn2+ on the inward rectifier potassium channel (IRK1) expressed in the Xenopus oocytes.

RESULTS

Mn2+ can concentration-, time- and vol-tage dependently block IRK1 instantaneous currents (2 ms after voltage applied). Mn2+ has almost no effect on the gating property of IRK1. IRK1 can not permeate Mn2+ because reverse potential did not changed. External Mn2+ can inhibit IRK1 macroscopic currents more powerfully when external Mn2+ concentration is lower and external Mn2+ can increases standard chord conductance of IRK1.

CONCLUSION

External Mn2+ works through surface potential mechanism. Ba2+ is considered as one fast open channel blocker of IRK1 and three exponential fitting results indicates that external Mn2+ can compete with Ba2+ in the same binding site in IRK1 when external Ba2+ concentration is 30 mumol/L. These mean two different mechanisms about external Mn2+ blocking exist.

[Effect of halothane on the muscarinic potassium current of the heart]

Acetylcholine (ACh) released from parasympathetic nerves binds to muscarinic (M2) ACh receptor (mAChR) in the heart, which leads to activation of muscarinic K(+) channel via the betagamma subunit of a G protein. The effect of a general anaesthetic (halothane) on the muscarinic K(+) channel (i(K,Ach)) in guinea-pig atrial cells was investigated using the whole-cell patch clamp technique. Halothane suppressed i(K,Ach), slowed down activation of i(K,Ach) and decreased peak i(K,Ach). When i(K,Ach) was activated by ACh acting via the muscarinic ACh receptors in the normal way, the decrease of i(K,Ach) was greater than the decrease when the muscarinic ACh receptor was bypassed and i(K,Ach) was activated by GTPgammaS. The above finding suggests that the suppression of i(K,Ach) by halothane is, in part, a result of the direct effect on the muscarinic K(+) channel or associated G protein. The decrease of i(K,Ach) by halothane may interfere with parasympathetic control of the heart.

[Blocking effects of extracellular Ba(2+) on the inward rectifier potassium channel]

Two-microelectrode voltage clamp (TEV) method was used to study the blocking effects of extracellular Ba(2+) on the inward rectifier potassium channel (IRK1) expressed in Xenopus oocytes. Blockage of Ba(2+) on IRK1 (1 ms after voltage applied) is Ba(2+) concentration (0,1,3,10 or 100 micromol/L) dependent with 10 or 90 mmol/L potassium and also voltage-dependent. Ba(2+) almost has no effect on the open/close of IRK1. IRK1 is not permeable to Ba(2+). Three exponential fitting analysis indicates that Ba(2+) and K(+) compete the same binding site in IRK1 when external Ba(2+) concentration is lower (1 or 3 micromol/L). The time constant of IRK1 does not increase, but the concentration dependency of the weights of the fittings increases with the increase of external Ba(2+) concentration. As a result, the inactivation becomes faster and faster as the external Ba(2+) concentration increases. Moreover, since the time constant of the channel decreases and the weights of the fittings concentration dependently increase with the increase of external Ba(2+) concentration (10 or 100 micromol/L), the inactivation becomes faster and faster. It is demonstrated that Ba(2+) can contact with deeper binding sites in IRK1 as external Ba(2+) concentration increases. It is suggested that two different mechanisms may underlie the external Ba(2+) blocking effect. External Mn(2+) or Mg(2+) can compete with external Ba(2+) at the IRK1 binding site at an external Ba(2+) concentration of 30 mol/L and K(+) concentration of 90 mmol/L. Inactivation becomes slower and slower and Ba(2+) is repelled from the IRK1 binding site when Mg(2+) or Mn(2+) con-centration is further increased. Mg(2+), but not Mn(2+), can contact with deeper binding sites of IRK1 to block the channel, suggesting that multiple-ion blockage may exist in IRK1.

[Comparison of vasorelaxing actions of vasonatrin peptide, C-type natriuretic peptide and atrial natriuretic peptide]

The vasorelaxing effects of vasonatrin peptide (VNP), C-type natriuretic peptide (CNP) and atrial natriuretic peptide (ANP) on isolated rat pulmonary artery, abdominal aorta and celiac vein were measured by in vitro perfusion. The results showed that VNP, CNP and ANP caused concentration-dependent relaxation in isolated rat pulmonary artery, abdominal aorta and celiac vein with endothelium or without endothelium. The maximal responses (Rmax) of VNP were (76 +/- 17)%, (51 +/- 14)% and (62 +/- 14)% in pulmonary artery, abdominal aorta and celiac vein with endothelium respectively, whereas those of CNP were (31 +/- 8)%, (22 +/- 7)% and (41 +/- 8)%, and ANP (38 +/- 10)%, (41 +/- 10)% and (11 +/- 4)%. The median effective concentration (EC50) of VNP were 16 +/- 11, 35 +/- 18 and 12 +/- 8 nmol/L in pulmonary artery, abdominal aorta and celiac vein with endothelium respectively, while those of CNP were 148 +/- 112, 299 +/- 84 and 14 +/- 12 nmol/L, and ANP 66 +/- 47, 16 +/- 15 and 909 +/- 445 nmol/L. VNP were more effective than CNP and ANP, and the differences were statistically significant (P < 0.05-0.01). The potency of these peptides for relaxing the blood vessels can be summarized as: VNP > ANP > or = CNP for pulmonary artery; VNP > ANP > CNP for abdominal aorta; VNP > CNP > ANP for celiac vein. There was no significant difference between vessels with intact endothelium and those denuded of endothelium (P > 0.05).

[Effects of monocarboxylic acid derivatives on cardiac ventricular CFTR Cl- channels in guinea pig]

Using the whole-cell recording technique, the effects of monocarboxylic acid derivatives on cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-)-channel were examined in guinea pig ventricular myocytes. Anthracene-9-carboxylic acid (9-AC) added to the bath solution further enhanced the outward component of isoproterenol-induced currents in a reversible manner, whereas 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) or diphenylamine-2-carboxylic acid (DPC) induced a biphasic effect on the currents. Either NPPB or DPC first produced a transient increase in the outward component of current before ensuing inhibition. Intracellular NPPB was found to potentiate isoproterenol-activated currents. It is concluded that these monocarboxylic acid derivatives have different binding sites in cardiac ventricular myocytes, which might partially account for the varied effects in blocking anion channels.

[The effect of batroxobin on coronary segmental resistance and coronary blood flow in acute myocardial ischemic dogs]

To study the effects of batroxobin on coronary circulation and cardiac performance in acute myocardial ischemia, Batroxobin was given intravenously to dogs with experimental coronary stenosis. A dose-dependent increase of coronary blood flow (CBF) was observed. Forty minutes after batroxobin (2 BU.kg-1 at infusion rate 0.1 BU.kg-1.min-1) administration, CBF increased by 12% (P < 0.05), small coronary resistance(RS) decreased from 4.1 +/- 0.5 to 3.2 +/- 0.5 mmHg.min.ml-1 (P < 0.01), while large coronary resistance(RL) changed insignificantly from 3.9 +/- 0.8 to 3.8 +/- 0.7 mmHg.min.ml-1 (P > 0.05). Two hours following drug administration, the changes in CBF, RS and RL still remained and RT decreased by 13% (P < 0.05). The + LV(dp/dt)max and -LV(dp/dt)max increased by 14% and 16% (P < 0.05) respectively compared with those in control group. It is concluded that batroxobin improves the ischemic canine coronary circulation and cardiac performance by way of lowering the small coronary resistance and thus increasing CBF. The data also suggest the benificial effect of batroxobin in acute myocardial ischemia.

[Study on pharmacokinetic characteristics of tetramethylpyrazine and hemodynamics of heart blood stasis in dogs]

"Blood stasis syndrome pharmacokinetics" means the pathophysiological state of blood stasis syndrome affectes significantly the pharmacokinetic parameters. In order to provide scientific evidence for the new hypothesis, the authors studied the hemodynamic changes induced by quantitative coronary artery stenosis and pharmacokinetics of tetramethylpyrazine (TMP) in dog model with heart blood stasis (HBS) and in normal dog (for control) simultaneously. Compared with healthy dog, serum TMP concentration, drug concentration time, area under curve (AUC) and t1/2 beta in HBS dog were increased significantly (P < 0.01). The mechanism might be related with the abnormal changes in hemodynamics (indude decrease in cardiac output, left ventricular systolic pressure and coronary blood flow, etc.) caused by coronary artery stenosis.

[Effect of nicorandil on coronary circulation in ischemic region]

To clarify the role of nicorandil (Nic) in relieving myocardial ischemia, the effects of Nic in canine models of coronary arterial stenosis were studied. During stenosis of the anterior descending coronary artery by an external micrometer constrictor, intracoronary infusion of Nic 1, 5 micrograms/(kg.min) increased coronary blood flow and decreased distal coronary pressure, total coronary arterial resistance and small coronary arterial resistance; but did not affect mean aortic pressure, heart rate and large coronary arterial resistance. Whole blood viscosity, plasma viscosity and hematocrit in coronary vein after intracoronary infusion of Nic were reduced. The results intravenous infusion of Nic were similar to those of intracoronary infusion. These results indicate that Nic is capable of increasing coronary blood flow, alleviating myocardial ischemia due to direct dilatation of coronary artery and reduction of blood viscosity in ischemic region.

[Effects of experimental coronary artery stenosis on platelet function]

In mild stenosis, coronary blood flow (CBF) was unchanged, thromboxane (TX) B2/6-ketoprostaglandin (PG) F1a ratio rose with no change in PAgT. In critical stenosis, CBF was slightly decreased, PAgT, TXB2 and TXB2/PGF1a ratio rose with cyclical reduction in CBF. In severe stenosis, CBF was markedly decreased; PAgT, TXB2 and TXB2/6-keto-PGF1a ratio rose and 6-keto-PGF1a decreased with cyclical blood flow reduction. Histopathologic examination confirmed the presence of damaged endothelial cell with coronary thrombosis and platelet/fibrin microemboli in critical and severe stenosis. It is concluded that coronary artery stenosis leads to a damage of endothelial cell, which causes an abnormality in platelet function and coronary thrombosis.

[The consumption of fibronectin content in coronary artery stenosis in dogs]

Changes in plasma fibronectin (Fn) content of the coronary sinus were observed in coronary artery stenosis of dogs. The results showed that coronary artery stenosis produced an acute inflammatory response and decreased Fn when coronary artery stenosis was more than 75%, which was associated with an increase in platelet aggregation and activation. Histopathologic examination confirmed the presence of damaged endothelial cell and capillary and showed the adhesion of leukocyte and platelet also. The results suggest that injured coronary artery may impair protective function in coronary circulation.

[Changes in platelet aggregation and coronary collateral circulation during the early phase of myocardial ischemia in dogs]

Experiments were performed on 18 anesthetized open-chest dogs to observe the changes in platelet aggregation and coronary collateral circulation during the early phase of acute myocardial ischemia. An increase in platelet aggregation rates (PAgR) and a decrease in platelet counts (PC) were found in the blood collected from the ischemic myocardium after coronary occlusion. PAgR was increased by 58.7 +/- 5.6% and PC was reduced by 39.5 +/- 23.6% at 50 min after occlusion (P less than 0.01). Under the condition of controlling aortic blood pressure, collateral coronary vascular capacity (CVC) was not changed (P greater than 0.05), but the effective collateral coronary flow to the ischemic zone, calculated by Wyatt et al's equation, was significantly reduced by 23.5 +/- 9.7% at 50 min after occlusion (P less than 0.05). There was a negative correlation between the changes in PAgR and the effective collateral coronary flow ( r = -0.857, P less than 0.01), and between the collateral indices and infarct size (r = -0.847, P less than 0.01). Abnormal changes in parameters of platelet and coronary collateral circulation after myocardial ischemia were nearly abolished by intravenous injection of aspirin before coronary occlusion. The results suggest that the deleterious changes of platelet aggregation during the early phase of the acute myocardial ischemia may decrease the effective collateral coronary flow significantly and thereby enlarge the infarct size.

[The relationship between coronary artery stenosis and myocardial metabolism of oxygen demand in dogs]

In open chest dogs, a micrometer constrictor was used to produce three different coronary artery stenosis (AS) on left circumflex coronary artery, and hemodynamics, blood gas and coronary sinus pH, lactate were then measured. In mild coronary AS, resting coronary artery flow (CBF) did not decrease; but lactate increased. In critical coronary AS, CBF slightly decreased, MVO2 decreased and lactate increased. There was a decrease in CBF, O2 supply/consumption ratio, coronary sinus pH and an increase in oxygen extraction and lactate with severe coronary AS. The percent area stenosis (%AS) was highly correlated with O2 supply (r = -0.92), poorly correlated with MVO2 (r = -0.58) when it was higher than 75%.

Alterations of coronary vascular responses to noradrenaline, acetylcholine and isoprenaline during acute myocardial ischaemia in dogs

1. The reactivity of coronary arteries to noradrenaline (NA), acetylcholine (ACh) and isoprenaline (Iso) during acute myocardial ischaemia was studied in anaesthetized, open-chest dogs. 2. The left anterior descending coronary artery (LAD) was cannulated and perfused via an extracorporal circuit at constant flow. Ischaemia was produced by decreasing the perfusion blood flow of the LAD to 50% (moderate ischaemia) and 27% (severe ischemia) of normal. A dose of each agent which had no significant myocardial effects was chosen. Heart rate was held constant by ventricular pacing during ACh administration. 3. Under normal conditions, the infusion of NA, ACh and Iso into the LAD produced significant decreases in coronary vascular resistance (P less than 0.01). However, under either moderate or severe ischaemic conditions intracoronary NA resulted in a marked increase in coronary vascular resistance (P less than 0.01), which was completely blocked by phentolamine, an alpha-adrenoceptor blocker; intracoronary ACh and Iso did not have a significant effect on coronary vascular resistance (P greater than 0.05). 4. These results indicate that coronary arterial responsiveness to some intrinsic vasoactive substances, such as NA and ACh, is altered during acute myocardial ischaemia. This may be important in the pathophysiology of myocardial ischaemia.

[Effects of stenosis on blood flow in the coronary artery of dogs]

The effects of stenosis on coronary blood flow (CBF) were studied in 22 open-chest mongrel dogs. A progressive stenosis was produced by a micrometer constrictor on the left circumflex coronary artery. Mean aortic pressure (Pa), distal coronary pressure (Pc) and stenotic segment pressure drop (delta P) were measured. Curve of CBF versus % stenosis showed that CBF was relatively constant with stenosis less than 85%, CBF rapidly decreased with stenosis of 85-95%, and then slowly reduced as stenosis was further increased. The change in CBF versus % stenosis was expressed as follows: CBF = 1.48 X 10(10) theta -27.6 A (where A = % stenosis in lumen area). With progressive coronary artery stenosis (greater than 75%), the change in percent stenosis and Pc showed a negative correlation: Pc = 159.1-1.36A (r = -0.73, P less than 0.01); the change in Pc and CBF showed a positive correlation: Pc = 16.9 + 1.3 CBF (r = 0.74, P less than 0.01).

[Effects of splenectomy on changes in hemorrheologic parameters following coronary occlusion in dogs]

Effects of splenectomy on changes of hemorrheologic parameters in the early stage of myocardial ischemia were observed in dogs. The results showed that splenectomy alleviated the increase in hematocrit and high shear viscosity of whole blood (r = 230 s-1) during 120 min of myocardial ischemia induced by coronary occlusion. Splenectomy reduced the increase in low shear viscosity of whole blood (r = 5.75 s-1) at 40 min and 80 min post-occlusion, but did not affect it at 120 min post-occlusion. Splenectomy had no effect on changes in blood viscosity fibrinogen concentration and plasma viscosity following coronary occlusion.

[Effect of cardiac sympathetic nerves on changes of TXB2 and 6-keto-PGF1 alpha in venous blood of ischemic myocardium]

Experiments were performed on 54 anesthetized, open-chest dogs to observe the effect of cardiac sympathetic nerves and alpha- and beta-adrenoceptor blockades on changes of platelet function during the early phase of myocardial ischemia. It was shown that contents of TXB2 and 6-keto-PGF1 alpha after coronary occlusion were markedly increased, and the platelet count was reduced in the coronary venous blood of ischemic myocardium. It was found that the changes in parameters of platelet function after coronary occlusion were significantly relieved by topical application of lignocaine-soaked gauze pads to the ischemic area of the heart or by bilateral stellectomy for blocking impulse of afferent and efferent of cardiac sympathetic nerves, compared with changes in parameters of control group, P less than 0.01. On the other hand, changes in parameters of the control group could be restored by intravenous infusion of noradrenaline to dogs which bilateral stellectomy was performed on, but not by infusion of normal saline. Different effects of alpha- and beta-adrenoceptor blockades on the changes in aforementioned parameters were found. The changing degrees in elevation of TXB2 and 6-keto-PGF1 alpha and in decrease of platelet count were significantly relieved by intravenous infusion of yohimbine (an alpha 2-blocker), phentolamine (a nonselective alpha-blocker), and propranolol (a beta-blocker), but not by infusion of prazosin (an alpha 1-blocker). Results were compared with the control group, P less than 0.01. These results suggest that cardiac sympathetic nerves play an important role for the changes of platelet function during the early phase of myocardial ischemia and that, yohimbine and phentolamine may exert an improving effect by blocking alpha 2-receptors on the platelet membrane, and propranolol may directly contribute to stabilization of the platelet membrane.