Influence of Na+-Independent Cl¯-HCO3¯ Exchange on the Slow Force Response to Myocardial Stretch

Previous work demonstrated that the slow force response (SFR) to stretch is due to the increase in calcium transients (Ca2+T) produced by an autocrine-paracrine mechanism of locally produced angiotensin II/endothelin activating Na+-H+ exchange. Although a rise in pHi is presumed to follow stretch, it was observed only in the absence of extracellular bicarbonate, suggesting pHi compensation through the Na+-independent Cl¯-HCO3¯ exchange (AE) mechanism. Because available AE inhibitors do not distinguish between different bicarbonate-dependent mechanisms or even between AE isoforms, we developed a functional inhibitory antibody against both the AE3c and AE3fl isoforms (anti-AE3Loop III) that was used to explore if pHi would rise in stretched cat papillary muscles superfused with bicarbonate after AE3 inhibition. In addition, the influence of this potential increase in pHi on the SFR was analyzed. In this study, we present evidence that cancellation of AE3 isoforms activity (either by superfusion with bicarbonate-free buffer or with anti-AE3Loop III) results in pHi increase after stretch and the magnitude of the SFR was larger than when AE was operative, despite of similar increases in [Na+]i and Ca2+T under both conditions. Inhibition of reverse mode Na+-Ca2+ exchange reduced the SFR to the half when the AE was inactive and totally suppressed it when AE3 was active. The difference in the SFR magnitude and response to inhibition of reverse mode Na+-Ca2+ exchange can be ascribed to a pHi-induced increase in myofilament Ca2+ responsiveness.Facultad de Ciencias MédicasCentro de Investigaciones Cardiovasculare

Influence of Na+-Independent Cl¯-HCO3¯ Exchange on the Slow Force Response to Myocardial Stretch

Previous work demonstrated that the slow force response (SFR) to stretch is due to the increase in calcium transients (Ca2+T) produced by an autocrine-paracrine mechanism of locally produced angiotensin II/endothelin activating Na+-H+ exchange. Although a rise in pHi is presumed to follow stretch, it was observed only in the absence of extracellular bicarbonate, suggesting pHi compensation through the Na+-independent Cl¯-HCO3¯ exchange (AE) mechanism. Because available AE inhibitors do not distinguish between different bicarbonate-dependent mechanisms or even between AE isoforms, we developed a functional inhibitory antibody against both the AE3c and AE3fl isoforms (anti-AE3Loop III) that was used to explore if pHi would rise in stretched cat papillary muscles superfused with bicarbonate after AE3 inhibition. In addition, the influence of this potential increase in pHi on the SFR was analyzed. In this study, we present evidence that cancellation of AE3 isoforms activity (either by superfusion with bicarbonate-free buffer or with anti-AE3Loop III) results in pHi increase after stretch and the magnitude of the SFR was larger than when AE was operative, despite of similar increases in [Na+]i and Ca2+T under both conditions. Inhibition of reverse mode Na+-Ca2+ exchange reduced the SFR to the half when the AE was inactive and totally suppressed it when AE3 was active. The difference in the SFR magnitude and response to inhibition of reverse mode Na+-Ca2+ exchange can be ascribed to a pHi-induced increase in myofilament Ca2+ responsiveness.Facultad de Ciencias MédicasCentro de Investigaciones Cardiovasculare

Influence of Na+-Independent Cl¯-HCO3¯ Exchange on the Slow Force Response to Myocardial Stretch

Previous work demonstrated that the slow force response (SFR) to stretch is due to the increase in calcium transients (Ca2+T) produced by an autocrine-paracrine mechanism of locally produced angiotensin II/endothelin activating Na+-H+ exchange. Although a rise in pHi is presumed to follow stretch, it was observed only in the absence of extracellular bicarbonate, suggesting pHi compensation through the Na+-independent Cl¯-HCO3¯ exchange (AE) mechanism. Because available AE inhibitors do not distinguish between different bicarbonate-dependent mechanisms or even between AE isoforms, we developed a functional inhibitory antibody against both the AE3c and AE3fl isoforms (anti-AE3Loop III) that was used to explore if pHi would rise in stretched cat papillary muscles superfused with bicarbonate after AE3 inhibition. In addition, the influence of this potential increase in pHi on the SFR was analyzed. In this study, we present evidence that cancellation of AE3 isoforms activity (either by superfusion with bicarbonate-free buffer or with anti-AE3Loop III) results in pHi increase after stretch and the magnitude of the SFR was larger than when AE was operative, despite of similar increases in [Na+]i and Ca2+T under both conditions. Inhibition of reverse mode Na+-Ca2+ exchange reduced the SFR to the half when the AE was inactive and totally suppressed it when AE3 was active. The difference in the SFR magnitude and response to inhibition of reverse mode Na+-Ca2+ exchange can be ascribed to a pHi-induced increase in myofilament Ca2+ responsiveness.Facultad de Ciencias MédicasCentro de Investigaciones Cardiovasculare

The Anrep effect requires transactivation of the epidermal growth factor receptor

Myocardial stretch elicits a biphasic contractile response: the Frank–Starling mechanism followed by the slow force response (SFR) or Anrep effect. In this study we hypothesized that the SFR depends on epidermal growth factor receptor (EGFR) transactivation after the myocardial stretch-induced angiotensin II (Ang II)/endothelin (ET) release. Experiments were performed in isolated cat papillary muscles stretched from 92 to 98% of the length at which maximal twitch force was developed (Lmax). The SFR was 123 ± 1% of the immediate rapid phase (n= 6, P < 0.05) and was blunted by preventing EGFR transactivation with the Src-kinase inhibitor PP1 (99 ± 2%, n= 4), matrix metalloproteinase inhibitor MMPI (108 ± 4%, n= 11), the EGFR blocker AG1478 (98 ± 2%, n= 6) or the mitochondrial transition pore blocker clyclosporine (99 ± 3%, n= 6). Stretch increased ERK1/2 phosphorylation by 196 ± 17% of control (n= 7, P < 0.05), an effect that was prevented by PP1 (124 ± 22%, n= 7) and AG1478 (131 ± 17%, n= 4). In myocardial slices, Ang II (which enhances ET mRNA) or endothelin-1 (ET-1)-induced increase in O2− production (146 ± 14%, n= 9, and 191 ± 17%, n= 13, of control, respectively, P < 0.05) was cancelled by AG1478 (94 ± 5%, n= 12, and 98 ± 15%, n= 8, respectively) or PP1 (100 ± 4%, n= 6, and 99 ± 8%, n= 3, respectively). EGF increased O2− production by 149 ± 4% of control (n= 9, P < 0.05), an effect cancelled by inhibiting NADPH oxidase with apocynin (110 ± 6%n= 7), mKATP channels with 5-hydroxydecanoic acid (5-HD; 105 ± 5%, n= 8), the respiratory chain with rotenone (110 ± 7%, n= 7) or the mitochondrial permeability transition pore with cyclosporine (111 ± 10%, n= 6). EGF increased ERK1/2 phosphorylation (136 ± 8% of control, n= 9, P < 0.05), which was blunted by 5-HD (97 ± 5%, n= 4), suggesting that ERK1/2 activation is downstream of mitochondrial oxidative stress. Finally, stretch increased Ser703 Na+/H+ exchanger-1 (NHE-1) phosphorylation by 172 ± 24% of control (n= 4, P < 0.05), an effect that was cancelled by AG1478 (94 ± 17%, n= 4). In conclusion, our data show for the first time that EGFR transactivation is crucial in the chain of events leading to the Anrep effect