To explore the mechanisms of adrenomedullin-induced vasorelaxation, we tested the effects of adrenomedullin on renal function in rats in vivo and measured the release of endothelium-derived nitric oxide from isolated perfused rat kidney (using a chemiluminescence assay) and the diameters of the glomerular arterioles in the hydronephrotic kidney. Adrenomedullin decreased blood pressure in a dose-dependent manner (3 nmol/kg: -29 +/- 2% [SEM]; P < .01) and slightly increased the glomerular filtration rate and urinary sodium excretion (+108%; P < .05). These changes were associated with significant increases in urinary excretion of cyclic AMP (+54%; P < .05). Adrenomedullin decreased renal vascular resistance (10(-7) mol/L adrenomedullin: -41 +/- 2%; P < .001) and increased release of nitric oxide (+5.1 +/- 0.7 fmol/min per gram kidney weight; P < .001) in the isolated kidney. This increase in nitric oxide release was abolished by the inhibitor NG-monomethyl-L-arginine, and it also reversed the decrease in renal vascular resistance seen with adrenomedullin. Renal responses of deoxycorticosterone acetate-salt hypertensive rats to adrenomedullin were significantly smaller than those of control rats for both release of nitric oxide (10(-7) mol/L adrenomedullin: +0.8 +/- 0.2 fmol/min per gram kidney weight; P < .01 versus control) and renal vasodilation (-28 +/- 6%; P < .05). Videomicroscopic analysis revealed that adrenomedullin increased the diameters of both afferent and efferent arterioles (3 nmol/kg: +11%; P < .05). Thus, adrenomedullin-induced renal vasodilation is partially endothelium dependent and is attenuated in deoxycorticosterone acetate-salt hypertension, probably due to endothelial damage.

1. Angiotensin II increases myocardial contractility in several species, including the rabbit and man. However, it is controversial whether the predominant mechanism is an increase in free cytosolic [Ca2+]i or a change in myofilament Ca"+ sensitivity. To address this question, we infused angiotensin II in isolated perfused rabbit hearts loaded with the Ca2" indicator indo-1 AM and measured changes in beat-to-beat surface transients of the Ca"+-sensitive 400 500 nm ratio and left ventricular contractility. The effects of angiotensin II were compared with the response to a Ca2+-dependent increase in the inotropic state produced by a change in the perfusate [Ca2"] from 0 9 to 3-6 mm.2. In the isolated beating heart, an increase in perfusate [Ca +] caused an increase in left ventricular pressure +dP/dt in association with an increase in peak systolic [Ca2+]1. Angiotensin II perfusion caused a similar increase in left ventricular +dP/dt in the absence of any increase in peak systolic [Ca2+]i.3. To exclude any contribution of non-myocyte sources of Ca2+-sensitive fluorescence which may be present in the intact heart, we also compared the effects of angiotensin II and a change in superfusate [Ca21] in collagenase-dissociated paced adult rabbit ventricular myocytes loaded with indo-1 AM. In the isolated rabbit myocytes a change in perfusate [Ca2+] from 0 9 to 3-6 mm caused an increase in peak systolic cell shortening coincident with an increase in peak systolic [Ca2+] Ca2"-force sensitivity secondary to intracellular alkalosis. METHODS Perfusion techniqueThe isolated and isovolumic heart perfusion system has been described in detail elsewhere ; Schunkert, Dzau, Tang, Hirsch, Apstein & Lorell, 1990). Male New Zealand White rabbits weighing 15-2-0 kg were given 500 units of heparin intravenously and were anaesthetized with 50 mg kg' pentobarbitone sodium. The hearts were isolated and coronary perfusion was initiated via a short cannula that was inserted into the aortic root within 30 s of opening the chest. Heart temperature was maintained at 37 'C. The hearts were perfused with a modified Krebs-Henseleit buffer of the following composition (mM): NaCl, 118; KCl, 4-7; CaCl2, 0'9; KH2PO4, 1-2; MgSO4, 1-2; NaHCO3 25; glucose, 5-5; and lactate, 1-0; with 0-1% fetal calf serum. The buffer was equilibrated with a gas mixture of 95% 02-5% CO2 which yields a partial pressure of 02 (PO2) of 580-620 mmHg and a pH of 7 35-7 45. After the initiation of oxygenated coronary perfusion with a constant flow pump, a small apical drain was placed to vent left ventricular Thebesian flow, and a pulmonary artery cannula was placed to completely collect coronary venous flow and empty the right ventricle. A collapsed latex balloon of slightly larger size than the left ventricular chamber was inserted into the left ventricle via the left atrium, and the venae cavae were ligated. Heart rate was paced and held constant at 3 Hz. Measurement of mechanical functionThe left ventricular balloon was filled with bubble-free saline and attached to a Stat...

AM has a negative inotropic effect and decreased both [Ca2+]i and Ica, with these effects being at least party mediated via the L-arginine-NO pathway in adult rabbit ventricular myocytes.

We examined the effects of endothelin-1 (ET-1) on intracellular free calcium concentration ([Ca2+]i) transients, intracellular pH (pHi), and cell contraction in both embryonic and neonatal as well as in adult ventricular myocytes. Exposure of chick ventricular myocytes to ET-1 (10 nM) significantly decreased both peak systolic and end-diastolic [Ca2+]i (from 949 +/- 43 to 628 +/- 59 nM and from 230 +/- 13 to 162 +/- 8 nM, respectively; P < 0.05, n = 12). The amplitude of cell contraction was also decreased during exposure to 10 nM ET-1 (81.7 +/- 1.2% of control, P < 0.01, n = 12). Exposure to 10 nM ET-1 slightly decreased pHi (-0.055 +/- 0.020 U; P < 0.05). Exposure of cultured neonatal rat ventricular myocytes to ET-1 (10 nM) produced similar effects. Responses of adult rabbit ventricular myocytes to ET-1 were dramatically different from those of embryonic or neonatal ventricular myocytes. Exposure to 10 nM ET-1 increased the amplitude of cell contraction to 159 +/- 32% of control (P < 0.01) without an increase in [Ca2+]i transients. ET-1 also increased pHi (+0.081 +/- 0.047 U; P < 0.01). These results indicate that ET-1 produces a negative inotropic effect by decreasing [Ca2+]i transients and induces a slight intracellular acidosis in immature ventricular myocytes. However, ET-1 causes a positive inotropic effect in adult ventricular myocytes via an intracellular alkalinization, rather than by an increase in the [Ca2+]i transient. Thus the response of myocytes to vasoactive peptides may vary with development and/or species.

To evaluate the effects of wall motion asynchrony on left ventricular (LV) relaxation, we performed atrioventricular sequential pacing with the second stimulation at six epicardial sites in open-chest anesthetized dogs. Myocardial segment lengths in the basal, mid, and apical LV free wall were measured by ultrasonic crystals. The standard deviation of interval from the onset of the QRS complex to that of elongation in each segment length was used as a quantitative index for asynchrony (asynchrony index, AI). The AI increased significantly in all sequential pacing modes compared with the control right atrial pacing. The time constant (T) of LV relaxation derived from exponential fit with zero-asymptote was prolonged significantly in all sequential pacing modes except for pacing at the LV base. In each dog there was a good correlation between changes in AI and T [r = 0.61 - 0.98 (mean = 0.84)]. Since the regional inactivation process of the myocardium is considered to be unchanged during these interventions, we concluded that asynchronous wall motion plays an important role in the impairment of LV relaxation.