Increased spontaneous activity and reduced inotropic response to catecholamines in ventricular myocytes from footshock-stressed rats

Figures & data

Table I.  Inotropic and automatic responses to isoproterenol (ISO) and norepinephrine (NE) in ventricular myocytes from control rats (CTR) and rats subjected to restraint (RST) and footshock (FS) for 3 days.

	ISO			NE
	CTR (17)	RST (13)	FS (17)	CTR (27)	RST (15)	FS (27)
Inotropic response
Rmax^† (% of RL)	11.2 ± 1.1	14.0 ± 2.0	5.4 ± 0.9^*,^*^*	14.8 ± 1.3	15.6 ± 1.7	8.4 ± 0.8^*,^*^*
pD2^‡	8.37 ± 0.07	8.28 ± 0.05	8.34 ± 0.14	7.55 ± 0.07	7.54 ± 0.08	7.85 ± 0.09^*,^*^*
Automatic response
Rmax^§ (SC/min)	4.4 ± 0.7	4.5 ± 0.7	10.3 ± 1.2^*,^*^*	5.3 ± 0.5	4.7 ± 0.6	10.2 ± 1.2^*,^*^*
pD2^‡	7.64 ± 0.08	7.42 ± 0.09	7.94 ± 0.18^*^*	6.77 ± 0.11	6.52 ± 0.10	6.92 ± 0.07^*^*

Data are presented as mean ± SEM. N (the number of studied cells) is shown in parentheses

^†maximum inotropic response (twitch amplitude as percentage of the resting cell length, RL)

^‡negative logarithm of the molar agonist concentration that evoked half-maximum response

^§maximum automatic response (rate of spontaneous contractions, SC, at rest)

^*p < 0.05 vs. CTR

^**p < 0.05 vs. RST (Bonferroni t-test).

Figure 1  Concentration-effect curves for norepinephrine (NE) in ventricular myocytes isolated from control rats (CTR; N = 27), as well as rats submitted to restraint (RST; N = 15) and footshock (FS; N = 27) sessions for 3 days. Inotropic response (panel A) was taken as the increase in peak twitch cell shortening (ΔL), expressed as percent of the resting cell length (RL). Automatic response (panel B) was taken as the increase in the rate of spontaneous contractions (SC) in the absence of electric stimulation. Symbols and bars indicate mean and SEM values, respectively. Curve parameters are presented in Table 1.

Table II.  Inotropic and automatic responses in ventricular myocytes from control (CTR) and footshock-stressed rats (FS): influence of β-adrenoceptor antagonists and response to 3-isobutyl-1-methylxanthine (IBMX).

Agon	Antag	CTR		FS
		Rmax^†	pD2	Rmax^†	pD2
Inotropic response (% of RL)
NE	PRZ	12.6 ± 1.9 (8)	7.45 ± 0.20	7.9 ± 1.1 (9)^*	7.92 ± 0.21^*
ISO	MET	10.5 ± 1.2 (6)	6.56 ± 0.11^*^*	9.5 ± 0.9 (10)^*^*	6.58 ± 0.13^*^*
ISO	BUT	7.9 ± 1.4 (16)	8.22 ± 0.15	5.8 ± 1.6 (10)	7.98 ± 0.20
ISO	ICI	8.1 ± 1.3 (22)	8.28 ± 0.13	7.2 ± 1.4 (15)	8.29 ± 0.11
IBMX	–	6.7 ± 0.9 (9)	5.04 ± 0.23	7.3 ± 0.7 (12)	4.93 ± 0.18
Automatic response (SC/min)
NE	PRZ	5.2 ± 0.8 (8)	6.72 ± 0.08	9.3 ± 0.9 (9)*	6.83 ± 0.25
ISO	MET	6.2 ± 1.2 (6)	5.87 ± 0.18^*^*	5.2 ± 0.6 (10)^*^*	6.20 ± 0.15^*^*
ISO	BUT	8.7 ± 0.8 (16)^*^*	7.48 ± 0.19	10.8 ± 2.4 (10)	7.49 ± 0.10
ISO	ICI	12.6 ± 1.0 (22)^*^*	7.53 ± 0.17	12.3 ± 1.1 (15)	7.42 ± 0.13
IBMX	–	6.8 ± 1.1 (9)	4.53 ± 0.20	6.4 ± 0.9 (10)	4.26 ± 0.11

Data are mean ± SEM. N (the number of studied cells) is shown in parenthesis

^‡Maximum inotropic (increase in twitch peak shortening, as percentage of the rest cell length, RL) and automatic responses (rate of spontaneous contractions, SC at rest). Agonists (Agon): norepinephrine (NE); isoproterenol (ISO). Antagonists (Antag): prazosin (PRZ, 0.1 μM); metoprolol (MET, 0.1 μM); butoxamine (BUT, 0.3 μM); and ICI118551 (ICI, 0.1 μM). For NE and ISO, data were also compared by two-way analysis of variance with those obtained in the absence of antagonists (shown in Table 1)

*p < 0.05 vs. CTR

^†p < 0.05 vs. in the absence of antagonist in the same group (Bonferroni t-test).

Figure 2  Concentration-effect curves for isoproterenol (ISO) in ventricular myocytes isolated from control (CTR) and footshock-stressed rats (FS), determined in the presence of 0.1 μM ICI118551 (ICI). Dashed gray and black lines indicate the curve to ISO obtained in the absence of the antagonist in CTR and FS, respectively. Twitch shortening peak (panel A) and rate of spontaneous contractions during rest (panel B) are expressed as in Figure 1. Panel C shows the rate of extrasystolic contractions (ES) recorded during pacing as a function of ISO concentration. Symbols and bars indicate mean and SEM values, respectively. Curve parameters are presented in Tables I and II.

Figure 3  Inotropic (left bars: increase in twitch peak shortening, ΔL, as percent of resting cell length, RL) and automatic (right bars: rate of spontaneous contractions, SC, at rest) responses to 3 μM forskolin (FSK) in ventricular myocytes isolated from control (CTR) and footshock-stressed (FS) rats. Bars are mean and SEM. Baseline ΔL was 8.0 ± 1.1 in CTR (N = 19) and 8.9 ± 0.8% of RL in FS (N = 20). Baseline SC rate was 0.3 ± 0.2 and 0.8 ± 0.3 SC/min in CTR and FS, respectively (p>0.10). ^*p < 0.05 vs CTR (t-test for unpaired samples).

Figure 4  Concentration-effect curves for 3-isobutyl-1-methylxanthine (IBMX) in ventricular myocytes isolated from control (CTR; N = 9) and footshock-stressed rats (FS; N = 12). Inotropic (panel A) and automatic (panel B) responses are expressed as in Figure 1. Symbols and bars indicate mean and SEM values, respectively. Baseline ΔL and SC rate were not significantly different in CTR and FS (p>0.36). Curve parameters are presented in Table II.

Figure 5  Inotropic (panel A) and automatic (panel B) responses to the increase in extracellular CaCl₂ in isolated ventricular myocytes from control (CTR; N = 8) and footshock-stressed rats (FS; N = 8). Symbols and bars indicate mean and SEM values, respectively.

Figure 6  A: Concentration-effect curve for isoproterenol (ISO) determined in the presence of 1 mM caffeine (caff). Inotropic and automatic responses were determined in the same set of control ventricular myocytes (N = 8) and are expressed as in Figure 1. Data are mean and SEM. Gray and black dashed lines indicate the curves for the inotropic and automatic effects of ISO, respectively, in the absence of caffeine. B: Relationship between automatic and inotropic maximum responses (R_max) to agonists determined in a set of 205 myocytes.