Stretch-Induced Alkalinization of Feline Papillary Muscle

Abstract

Abstract-Myocardial stretch is a well-known stimulus that leads to hypertrophy. Little is known, however, about the intracellular pathways involved in the transmission of myocardial stretch to the cytoplasm and nucleus. Studies in neonatal cardiomyocytes demonstrated stretch-induced release of angiotensin II (Ang II). Because intracellular alkalinization is a signal to cell growth and Ang II stimulates the Na ϩ /H ϩ exchanger (NHE), we studied the relationship between myocardial stretch and intracellular pH (pH i ). Experiments were performed in cat papillary muscles fixed by the ventricular end to a force transducer. Muscles were paced at 0.2 Hz and superfused with HEPES-buffered solution. pH i was measured by epifluorescence with the acetoxymethyl ester form of the pH-sensitive dye 2Ј,7Ј-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF-AM). Each muscle was progressively stretched to reach maximal developed force (L max ) and maintained in a length that was Ϸ92% L max (L i ). During the "stretch protocol," muscles were quickly stretched to L max for 10 minutes and then released to L i ; pH i significantly increased during stretch and came back to the previous value when the muscle was released to L i . The increase in pH i was eliminated by (1) Key Words: stretch, myocardial Ⅲ pH, intracellular Ⅲ Na ϩ /H ϩ exchange Ⅲ angiotensin Ⅲ endothelin A lthough it is well known that mechanical stimuli cause a variety of effects on the structure and function of the myocardial cells, little is known about how cells sense the mechanical stimuli, transmit the information to messenger systems, and finally regulate function and growth.1,2 Highly regarded experiments demonstrate that the release of angiotensin II (Ang II) contributes to stretch-induced hypertrophy in cultured neonatal cardiac myocytes, [2][3][4][5] and that the effect was suppressed by the AT 1 -receptor antagonist TCV-116. 4 The release of Ang II may involve an autocrine or paracrine mechanism because stretch-conditioned media mimicked the effect of stretch when transferred to nonstretched neonatal cardiomyocytes.2 An increase in PKC activity is an effect detected after stretching cultured neonatal cardiomyocytes 2 and the adult heart. 6 Because Ang II, by mechanisms still unresolved but probably linked to PKC, activates the Na ϩ /H ϩ exchanger (NHE), 7-8 the logical expectation is a rise in intracellular pH (pH i ) after myocardial stretch. Although we are unaware of measurements of myocardial pH i before and during stretch, pressure overload increased NHE-1 mRNA levels in hearts.9 Furthermore, the activity of mitogen-activated protein kinase (MAP kinase) was increased by stretch in cultured cardiomyocytes, and this increase was partially eliminated by NHE inhibition.9 Nevertheless, an unknown is whether stretch alters NHE activity in multicellular preparations from adult hearts. This point is critical, because Ang II released after stretch might increase NHE activity 10 -14 and promote the expression of endothelin (ET) as well as the upregulation of ET rec...

Myocardial Mineralocorticoid Receptor Activation by Stretching and Its Functional Consequences

“…Alkalization of cardiac muscle in bicarbonate-free medium is a key signal of stretch-triggered NHE1 activation 12 ; therefore, we next determined pH i changes after stretching isolated papillary muscles from both experimental groups. As expected, a significant increase in pH i was detected in papillary muscles from control-injected hearts, whereas no pH i changes were observed in the l-shMR-injected group as shown in Figure 4, revealing the absence of stretch-induced NHE1 activation.…”

Costameres, focal adhesions, and cardiomyocyte mechanotransduction

“…Since the first description of the rapid release of preformed angiotensin II into the culture medium of cultured neonatal rat ventricular myocytes subjected to static stretch (103), several studies have documented that mechanical load causes the increased expression and release of a variety of factors, including angiotensin II (75,103,113), endothelin-1 (141), VEGF (108), TGF-␤ (100), and cardiotrophin-1 (81) by cultured neonatal cardiomyocytes. There is also some evidence to suggest that acute mechanical loading rapidly releases similar growth factors and cytokines from adult cardiomyocytes (20,64). However, few studies have addressed the responsible mechanisms involved in transducing mechanical signals into biochemical signals that regulate growth factor expression and secretion by cardiomyocytes.…”

Cardiac Basal Metabolism.

“…It is now well-recognized that a stretch of cultured myocytes activates several independent biochemical cascades whose final common pathway involves the sarcolemmal Na ϩ -H ϩ exchanger (for recent reviews see [199,200]). Thus stretch releases angiotensin II [201], which, probably via a paracrine or autocrine mechanism [202] involving endothelin [203] and possibly the protein kinase-C pathway [203,204], activates an Na ϩ -H ϩ exchange leading to an increase of intracellular pH [205]. It is interesting to note that the duration of the stretch-induced change of pH (usually several minutes in a multicellular preparation from adult rat hearts [205]) is comparable to that of the stretch-induced increase of resting heat rate in the same species [64].…”

“…Thus stretch releases angiotensin II [201], which, probably via a paracrine or autocrine mechanism [202] involving endothelin [203] and possibly the protein kinase-C pathway [203,204], activates an Na ϩ -H ϩ exchange leading to an increase of intracellular pH [205]. It is interesting to note that the duration of the stretch-induced change of pH (usually several minutes in a multicellular preparation from adult rat hearts [205]) is comparable to that of the stretch-induced increase of resting heat rate in the same species [64]. Nevertheless, this temporal similarity may be merely fortuitous since the longitudinal stretch of cultured myocytes, even by as much as 10% of cell length, has no effect on the turnover of the total pool of contractile proteins or on either its cytoplasmic or contractile matrix components, but the concentrations of both actin and myosin heavy chains remain constant [206].…”