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Subcellular mechanisms of the positive inotropic effect of angiotensin II in cat myocardium

Martín G. Vila Petroff, Ernesto A. Aiello, Julieta Palomeque, Margarita A. Salas and Alicia Mattiazzi

1. Cat ventricular myocytes loaded with [Ca²⁺]_i- and pH_i-sensitive probes were used to examine the subcellular mechanism(s) of the Ang II-induced positive inotropic effect. Ang II (1 µM) produced parallel increases in contraction and Ca²⁺ transient amplitudes and a slowly developing intracellular alkalisation. Maximal increases in contraction amplitude and Ca²⁺ transient amplitude were 163 ± 22 and 43 ± 8 %, respectively, and occurred between 5 and 7 min after Ang II administration, whereas pH_i increase (0·06 ± 0·03 pH units) became significant only 15 min after the addition of Ang II. Furthermore, the inotropic effect of Ang II was preserved in the presence of Na⁺-H⁺ exchanger blockade. These results indicate that the positive inotropic effect of Ang II is independent of changes in pH_i.

2. Similar increases in contractility produced by either elevating extracellular [Ca²⁺] or by Ang II application produced similar increases in peak systolic Ca²⁺ indicating that an increase in myofilament responsiveness to Ca²⁺ does not participate in the Ang II-induced positive inotropic effect.

3. Ang II significantly increased the L-type Ca²⁺ current, as assessed by using the perforated patch-clamp technique (peak current recorded at 0 mV: -1·88 ± 0·16 pA pF^-1 in control vs. -3·03 ± 0·20 pA pF^-1 after 6-8 min of administration of Ang II to the bath solution).

4. The positive inotropic effect of Ang II was not modified in the presence of either KB-R7943, a specific blocker of the Na⁺-Ca²⁺ exchanger, or ryanodine plus thapsigargin, used to block the sarcoplasmic reticulum function.

5. The above results allow us to conclude that in the cat ventricle the Ang II-induced positive inotropic effect is due to an increase in the intracellular Ca²⁺ transient, an enhancement of the L-type Ca²⁺ current being the dominant mechanism underlying this increase.

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