Intraventricular and interventricular cellular heterogeneity of inotropic responses to α(1)-adrenergic stimulation

Abstract

α1-Adrenergic receptors (α1-ARs) elicit a negative inotropic effect (NIE) in the mouse right ventricular (RV) myocardium but a positive inotropic effect (PIE) in the left ventricular (LV) myocardium. Effects on myofilament Ca(2+) sensitivity play a role, but effects on Ca(2+) handling could also contribute. We monitored the effects of α1-AR stimulation on contraction and Ca(2+) transients using single myocytes isolated from the RV or LV. Interestingly, for both the RV and LV, we found heterogeneous myocyte inotropic responses. α1-ARs mediated either a PIE or NIE, although RV myocytes had a greater proportion of cells manifesting a NIE (68%) compared with LV myocytes (36%). Stimulation of a single α1-AR subtype (α1A-ARs) with a subtype-selective agonist also elicited heterogeneous inotropic responses, suggesting that the heterogeneity arose from events downstream of the α1A-AR subtype. For RV and LV myocytes, an α1-AR-mediated PIE was associated with an increased Ca(2+) transient and a NIE was associated with a decreased Ca(2+) transient, suggesting a key role for Ca(2+) handling. For RV and LV myocytes, α1-AR-mediated decreases in the Ca(2+) transient were associated with increased Ca(2+) export from the cell and decreased Ca(2+) content of the sarcoplasmic reticulum. In contrast, for myocytes with α1-AR-induced increased Ca(2+) transients, sarcoplasmic reticulum Ca(2+) content was not increased, suggesting that other mechanisms contributed to the increased Ca(2+) transients. This study demonstrates the marked heterogeneity of LV and RV cellular inotropic responses to stimulation of α1-ARs and reveals a new aspect of biological heterogeneity among myocytes in the regulation of contraction.

Fig. 1.

Baseline recording of Ca²⁺ transients and contractions of right ventricular (RV) myocytes (n = 31) and left ventricular (LV) myocytes (n = 28). A: diastolic and systolic Ca²⁺ levels. B: sarcomere length. C: time constant (τ) of Ca²⁺ decline. D: τ of relaxation of contraction. **P < 0.01; ***P < 0.001.

Fig. 2.

Slow time-based recordings of electrically stimulated contractions before and after stimulation of α₁-adrenergic receptors (α₁-ARs) with 10 μM phenylephrine (PE) plus the β-blocker timolol (10 μM). A: addition of PE (arrow) elicited a negative inotropic effect (NIE) in some RV cells. B: PE elicited a positive inotropic effect (PIE) in other RV cells. C and D: LV cells also manifest a NIE or PIE after α₁-AR stimulation. E: there was in no inotropic response to treatment with the vehicle (Veh) control.

Fig. 3.

Summary of α₁-AR inotropic responses of RV and LV myocytes (expressed as the percent change from the baseline contraction level for each cell). Heterogeneity of inotropic responses was observed using the nonsubtype-selective α₁-AR agonist PE (A) and also using the α_1A-AR subtype-selective agonist A-61603 (B).

Fig. 4.

Examples of a NIE and PIE elicited by α₁-ARs in LV and RV myocytes. Records of fura-2-induced Ca²⁺ transients and cell contractions (assessed from sarcomere length measures) are shown. Compared with before the addition of PE, α₁-AR stimulation caused decreased Ca²⁺ transients and decreased contractions in some cells (A and C) or increased Ca²⁺ transients and contraction in other cells (B and D).

Fig. 5.

Relationship between the amplitude of myocyte contraction and Ca²⁺ transient amplitude for RV cells (open symbols) and LV cells (solid symbols). Data are shown for cells before the addition of PE (●) and after 10 min in the presence of PE for cells that manifested a PIE (▲) or a NIE (▼).

Fig. 6.

Effects on α₁-AR stimulation on Ca²⁺ handling as assessed with caffeine-induced contractures. A: records of caffeine-induced Ca²⁺ transients in LV cells before the addition of PE and 10 min after PE induced either a NIE or a PIE. B: summary of caffeine-induced Ca²⁺ transients before (Con) and after α₁-AR stimulation induced a NIE or PIE in RV or LV cells. C: summary of the time course of decline of the caffeine-induced Ca²⁺ transient as assessed from the relaxation half-time (RT₅₀; same groups as in B). RV and LV myocyte numbers were 17 and 14 for Con, 8 and 8 for NIE, and 9 and 7 for PIE, respectively. *P < 0.05; ***P < 0.001.