Effects of angiotensin II on intracellular Ca2+ and pH in isolated beating rabbit hearts and myocytes loaded with the indicator indo‐1.

Abstract

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...

Inhibition of Agonist-Induced Positive Inotropy by a Selective Rho-Associated Kinase Inhibitor, Y-27632

“…-sensitivity in cardiac muscle (1,5). One of the explanations of the present study is that Rho/ ROCK pathways also regulate the Ca…”

“…Although earlier reports suggest the involvement of increased Ca 2+ influx through L-type Ca 2+ channels (3,4) and increase in Ca 2+ sensitivity of myofibrils (5), the intracellular mechanisms leading to these changes have not yet been clarified. These vasoconstrictive agonists are also known to play a role in the development of cardiac hypertrophy or heart failure (6,7).…”

“…-sensitization of cardiac muscle is caused by the intracellular alkalization resulting from activation of Na + / H + exchanger (4,5). It was also reported that activation of the Rho / ROCK pathway could increase (15).…”

Effects of acute angiotensin II on ischemia reperfusion injury following myocardial infarction

“…These second messengers subsequently promote inotropy by increasing sarcolemmal I Ca 2+ and [Ca 2+ ] I , and by alkalinizing cardiomyocytes via protein kinase C (PKC) activation of the Na + /H + exchanger. 7,[13][14][15][16] The role of AT 2 receptors is less clear, but they have been shown to act in vasodilation, in part through nitric oxide-cGMP signaling. 7 We and others have shown that angiotensin II improves inotropy in normal rat hearts during conditions of acidosis.…”

General – Heart and Cardiac Muscle

“…In single cells, AIl may increase cl+ via direct modulation of the L-type channels (Baker et al 1989). However, other studies have shown that the inotropic effect is secondary to an intracellular alkalosis and increased myofilament Ca 2 + sensitivity (Ikenouchi et al 1994). Our aim was to determine whether the functional response to AIl stimulation in the intact, beating heart was caused by a significant change in intracellular pH (pHi)' New Zealand White rabbits were anaesthetized using Hypnorm (0'3 ml (kg body wtfi, LM .)…”