Emotional stress affects cellular integrity in many tissues including the heart. Much less is known about the effects of social stress. We studied the effect of emotional (immobilization with or without cold exposure) or social (intermale confrontation) stress in mice. Tissue injury was measured by means of the release of enzyme activities to blood plasma: lactate dehydrogenase (LDH), creatine kinase (CK), aspartate transaminase (AST), and alanine transaminase (ALT). Tape-immobilization increased all these activities in the plasma. AST-ALT ratio was also increased in these animals. Electrophoretic analysis of CK isoenzymes showed the appearance of CK-MB. These results indicate that the heart was injured in immobilized mice. Analysis of LDH isoenzymes and measurement of alpha-hydroxybutyrate dehydrogenase (HBDH) activity suggests that other tissues, in addition to the heart, contribute to the increase in plasma LDH activity. Restraint in small cylinders increased plasma LDH, CK, AST, and ALT activities, but to lower levels than in tape immobilization. Because the decrease in liver glycogen and the increase in plasma epidermal growth factor (EGF) were also smaller in restraint than in the tape-immobilization model of emotional stress, we conclude that the former is a less intense stressor than the latter. Cold exposure during the restraint period altered the early responses to stress (it enhanced liver glycogen decrease, but abolished the increase in plasma EGF concentration). Cold exposure during restraint enhanced heart injury, as revealed by the greater increase in CK and AST activities. Intermale confrontation progressively decreased liver glycogen content. Plasma EGF concentration increased (to near 100 nM from a resting value of 0.1 nM) until 60 minutes, and decreased thereafter. Confrontation also affected cellular integrity in some tissues, as indicated by the rise in plasma LDH activity. However, in this type of stress, the heart appeared to be specifically protected because there was no increase in plasma CK activity, and both AST and ALT increased, but the AST-ALT ratio remained constant. Habituation to restraint (1 h/d, 4 days) made mice resistant to restraint-induced tissue injury as indicated by the lack of an increase in plasma LDH, CK, AST, or ALT activities. Similar general protection against homotypic stress-induced injury was observed in mice habituated to intermale confrontation.

Triacylglycerol (TG) hydrolase activities were characterized in myocytes isolated from rat hearts. Acid hydrolase activity with a pH optimum of 5 could be measured in myocyte homogenates, and the subcellular distribution suggested that this activity originated in lysosomes. Lipoprotein lipase (LPL) was also present in myocyte homogenates, as evidenced by TG hydrolase activity that was stimulated by serum and apolipoprotein CII, and inhibited by apolipoprotein CIII2, high ionic strength (NaCl and MgCl2, I = 1 M) and antibodies to LPL. Serum-independent neutral (pH 7.5) TG hydrolase activity was less sensitive to inhibition by 1 M-NaCl, by antibodies to LPL and by preincubation at 40 degrees C than was serum-stimulated hydrolase activity. Furthermore, there were modest but significant differences in the subcellular distribution of the serum-independent and serum-stimulated hydrolase activities. Hydrolase activities in myocyte homogenates could be solubilized by 7.2 mM-deoxycholate. Acid hydrolase activity was recovered in the unbound fraction after heparin-Sepharose chromatography, whereas LPL was bound to the affinity column and was eluted by 0.9-1.2 M-NaCl. Approximately one-third of the serum-independent TG hydrolase activity was not bound to the heparin-Sepharose affinity column. This unbound TG hydrolase activity had a pH optimum of 7 and was stimulated by 50 mM-MgCl2, but not by serum and was resistant to inhibition by high ionic strength (1 M-NaCl), to preincubation at 40 degrees C for 2 h, and by antibodies to LPL. It is concluded that, in addition to an acid lysosomal TG hydrolase and LPL, myocytes from rat heart contain a serum-independent TG hydrolase with unique characteristics.

The epidermal growth factor (EGF) concentration in bile is high (approximately 150 fold higher than that in plasma), but little is known about its physiological control. Acute administration of the alpha 1-adrenergic agonist phenylephrine (1.7 mg/kg, iv) to male mice produced a rapid increase in the EGF concentration in bile. We suggest that this EGF originates in submandibular glands and not in the liver. The bases for this are: 1) this increase was parallel to the increase in plasma, and the EGF content of the submandibular glands decreased after phenylephrine injection; and 2) the EGF concentrations in plasma and bile did not increase after phenylephrine administration to sialoadenalectomized mice. The concentration of EGF in bile is not only under pharmacological control, but is also regulated physiologically. Thus, the EGF concentrations in plasma, bile, and submandibular glands increased in fasted mice. All of these changes were reversed by refeeding. As 1) [125I]EGF binding to liver membranes decreased only after 2 days of fasting, but the level of circulating EGF was already increased in 1-day fasted mice, and 2) EGF secretion by submandibular glands from 1-day fasted mice incubated in vitro increased, we suggest that the increase in EGF concentrations in plasma and bile is the consequence of increased endocrine secretion by submandibular glands. Taken together, our results suggest that there is a flux of EGF from submandibular glands to bile in mice, which is under physiological control.

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Acute, high-intensity stress induces necrotic lesions in the heart. We found that restraint-and-cold (4°C) exposure (RCE) raises plasma lactate dehydrogenase (LDH), creatine kinase (CK), and transaminase activity in a time-dependent manner, with a peak value 7 h after stimulus cessation. At 24 h, signs of necrotic lesions were observed in paraffin sections stained with hematoxylineosin: focal accumulation of mononuclear cells in subendocardial areas of the left ventricle wall and focal hemorrhage in papillary muscles. In contrast, intermale fighting (IF) did not increase plasma CK activity, although LDH and transaminase activities did increase. In IF, no histological evidence of heart injury was observed. Because IF, but not RCE, increased plasma epidermal growth factor (EGF) concentration by ∼1,000-fold, we hypothesized that EGF receptor (ErbB1) activation may protect the heart against stress-induced injury. To examine this hypothesis, we injected the ErbB1 tyrosine kinase inhibitor tyrphostin AG-1478 (25 mg/kg ip) immediately before mice were exposed to IF. After 3 h, plasma activities of LDH-1 and CK increased. Plasma enzyme activities were as low in control mice (injected with vehicle alone) as in nonfighting mice. In the last experiment, we injected EGF (0.25 mg/kg ip) 20 min before exposing mice to RCE. After 7 h, plasma LDH-1 and CK activities were significantly lower in these animals than in mice injected with vehicle. The effect required ErbB1 activation, because simultaneous administration of AG-1478 completely abolished the effect of exogenous EGF. We conclude that activated ErbB1, by endogenous or exogenous ligands, may protect the heart against stress-induced injury.