Autonomic nervous control of myoepithelial cells and secretion in submandibular gland of anaesthetized dogs

Abstract

In dog submandibular gland, the activity of myoepithelial cells was assessed by simultaneous measurement of intraductal pressure (P(du)) and subcapsular pressure (P(ca)) using catheter-tip pressure transducers; their resting values were 2.5 +/- 0.21 and 3.0 +/- 0.19 mmHg, respectively (n = 40). Retrograde infusion of saliva (collected from preceding parasympathetic nerve stimulation) increased P(du) (coefficient of 50 mmHg ml(-1) for rates < 1 ml min(-1) and 85 mmHg ml(-1) for higher rates) and P(ca) (coefficient of 0.47 mmHg ml(-1) for all rates). Blood flow changes did not affect P(du) but increased P(ca) (coefficient of 0.04 mmHg ml(-1)). Parasympathetic nerve stimulation increased P(du) but decreased P(ca) abruptly; the response threshold was 0.1 Hz, with maximal responses at 16 Hz. The coefficients for P(du) and P(ca) on salivary secretion to parasympathetic nerve stimulation in glands with spontaneous blood flow (5.3 x 10(-3) and 4.87 x 10(-2) ml min(-1) g(-1) mmHg(-1)) were close to their values in glands with constant-flow vascular perfusion (4.9 x 10(-3) and 3.68 x 10(-2) ml min(-1) g(-1) mmHg(-1)). The finding that P(ca) fell despite concomitant increased blood flow suggests contraction of myoepithelial cells. Additional ductal occlusion further increased P(du) and enhanced the fall in P(ca), suggesting that the myoepithelial cells can contract when distended. Atropine blocked salivary secretion and responses of P(du) and P(ca) to parasympathetic nerve stimulation. ACh elicited responses similar to that of parasympathetic nerve stimulation. VIP caused very scanty salivary secretion and gradual slight increases in P(du) and P(ca); the change in P(ca) was abolished in glands with constant-flow vascular perfusion. Hence, contraction of myoepithelial cells to parasympathetic nerve stimulation is via muscarinic receptors. Sympathetic nerve stimulation increased P(du) and decreased P(ca) abruptly; the response threshold was 0.1 Hz, with maximal responses at 16 Hz. The coefficients for P(du) and P(ca) on salivary secretion to sympathetic nerve stimulation in glands with spontaneous blood flow (3.0 x 10(-3) and 3.2 x 10(-3) ml min(-1) g(-1) mmHg(-1)) were similar to their values in glands with constant-flow vascular perfusion (3.2 x 10(-3) and 3.1 x 10(-3) ml min(-1) g(-1) mmHg(-1)). The finding that P(ca) fell even in glands with constant-flow vascular perfusion suggests contraction of myoepithelial cells. Superimposed sympathetic nerve stimulation immediately enhanced the pressure changes and secretory response to parasympathetic nerve stimulation, indicating that the two autonomic nerves act synergistically to evoke myoepithelial cell contraction. Phentolamine and prazosin but not propranolol and yohimbine blocked the sympathetic enhancement. The finding that phenylephrine, but not clonidine and isoproterenol, abruptly decreased P(ca) in glands with constant-flow vascular perfusion suggests that the sympathetic activation of myoepithelial cells is via the alpha(1)-adrenoceptors.

Figure 1. Method of vascular perfusion of the submandibular gland and measurement of salivary flow (), intraductal pressure (P_du) and subcapsular pressure (P_ca) (A) and diagram showing the placement of the catheter-tip pressure transducer for measuring P_ca (B)

Sg, submandibular gland; Sd, submandibular duct; Fc, fibrous capsule; Dr, drop-counter; Fa, facial artery; Ga, glandular artery; Jv, jugular vein; Gv, glandular vein; Pt(1), catheter-tip pressure transducer for measuring P_ca; Pt(2), catheter-tip pressure transducer for measuring P_du; S, snare; Ppa, glandular arterial perfusion pressure; P, peristaltic pump; R, blood reservoir maintained at 37°C; Fea, connection to femoral artery; D, site of drug administration; Ptm, membrane sensor; Ptc, cable of the transducer.

Figure 2. The effects of retrograde infusion of saliva on P^du (•) and P_ca (▪) in glands with a closed ductal system at rest

n = 4.

Figure 3. The effects of changes in blood flow on P_du (•) and P_ca (▪) in glands at rest

n = 4.

Figure 4. Experimental recordings showing changes in arterial blood flow () or perfusion pressure (P_pa), P_du, P_ca and in response to parasympathetic nerve stimulation in glands with spontaneous blood flow (A) and glands with constant-flow vascular perfusion at normal flow rate (B)

Bar, stimulation; BP, systemic arterial blood pressure

Figure 5. Effects of parasympathetic nerve stimulation on P_du (A) and P_ca (B) in normal and atropinized glands and relationship between the steady state and the change in P_du (C) and P_ca (D) during parasympathetic nerve-induced salivation

A, peak (•, n = 8), steady state (<$$>, n = 8) and atropinized steady state (⊙, n = 4) responses in glands with spontaneous blood flow. Peak (▾, n = 8), steady state (▽, n = 8) and atropinized steady state (, n = 4) responses in glands with constant-flow vascular perfusion at normal flow rate. B, peak (▪, n = 8), steady state (□, n = 8) and atropinized steady state (, n = 4) responses in glands with spontaneous blood flow. Peak (♦, n = 8), steady state (⋄, n = 8) and atropinized steady state (, n = 4) responses in glands with constant-flow vascular perfusion at normal flow rate. C, the regression coefficient for P_du on is 5.3 (± 0.28) × 10⁻³ ml min⁻¹ g⁻¹ mmHg⁻¹ (r = 0.99, n = 5, P < 0.001) for glands with spontaneous blood flow (^) and is 4.9 (±0.29) × 10⁻³ ml min⁻¹ g⁻¹ mmHg⁻¹ (r = 0.99, n = 5, P < 0.001) for glands with constant-flow vascular perfusion at normal flow rate (∇).D, the regression coefficient for P_ca on is 4.87 (± 0.20) × 10⁻² ml min⁻¹ g⁻¹ mmHg⁻¹ (r = 0.99, n = 5, P < 0.001) for glands with spontaneous blood flow (□) and is 3.68 (± 0.42) × 10⁻² ml min⁻¹ g⁻¹ mmHg⁻¹ (r = 0.99, n = 5, P < 0.001) for glands with constant-flow vascular perfusion at normal flow rate (⋄). For A and B, * P < 0.05, when compared to corresponding control. + P < 0.05, when compared to corresponding normal response.

Figure 6. Experimental recordings showing the effect of ductal occlusion during parasympathetic nerve stimulation in a gland with constant-flow vascular perfusion at normal flow rate

Continuous bar, stimulation at 2 Hz; dotted bar, ductal occlusion.

Figure 7. The effects of autonomic mimetics on P_du, P_ca and in glands with spontaneous blood flow (A) and glands with constant-flowvascular perfusionat normal flow rate (B)

a, ACh; v, VIP; p, phenylephrine; c, clonidine; i, isoproterenol. * P < 0.05, when compared to corresponding control; n = 6.

Figure 8. Experimental recordings showing changes in blood flow () or perfusion pressure (P_pa), P_du P_ca and to sympathetic nerve stimulation in glands with spontaneous blood flow (A) and glands with constant-flow vascular perfusion (B)

Bar, stimulation.

Figure 9. Effects of sympathetic nerve stimulation on P_du (A) and P_ca (B) and the relationship between the steady state and the change in P_du (C) and in P_ca (D) during sympathetic nerve-induced salivation

A, peak (•) and steady state (^) responses in glands with spontaneous blood flow; peak () and steady state (∇) responses in glands with constant-flow vascular perfusion. B, peak (▪) and steady state (□) responses in glands with spontaneous blood flow; peak (♦) and steady state (⋄) responses in glands with constant-flow vascular perfusion at normal flow rate. For A and B, * P < 0.05, when compared to corresponding control; n = 8. C, the regression coefficient for P_du on is 3.0 (± 0.29) × 10⁻³ ml min⁻¹ g⁻¹ mmHg⁻¹ (r = 0.99, n = 5, P < 0.001) for glands with spontaneous blood flow (^) and is 3.2 (± 0.38) × 10⁻³ ml min⁻¹ g⁻¹ mmHg⁻¹ (r = 0.99, n = 5, P < 0.001) for glands with constant-flow vascular perfusion at normal flow rate (∇). D, the regression coefficient for P_ca on is 3.2 (±0.34) × 10⁻³ ml min⁻¹ g⁻¹ mmHg⁻¹ (r = 0.99, n = 5, P < 0.001) for glands with spontaneous blood flow (□) and is 3.1 (± 0.49) × 10⁻³ ml min⁻¹ g⁻¹ mmHg⁻¹ (r = 0.99, n = 5, P < 0.001) for glands with constant-flow vascular perfusion at normal flow rate (⋄).

Figure 10. Experimental recordings illustrating the actions of adrenoceptor blockers on the effects of superimposed sympathetic nerve stimulation on parasympathetic nerve-induced changes in P_du, P_ca and in a gland with constant-flow vascular perfusion at normal flow rate

A, normal response. B, after yohimbine (0.025 mg kg⁻¹). C, after yohimbine (0.025 mg kg⁻¹) and prazosin (0.025 mg kg⁻¹). Continuous bar, parasympathetic nerve stimulation at 1 Hz; dotted bar, sympathetic nerve stimulation at 20 Hz.

Figure 11. Effects of superimposition of sympathetic nerve stimulation and adrenergic antagonists on the responses of P_du and P_ca to parasympathetic nerve stimulation (at 1 Hz) in glands with constant-flow vascular perfusion at normal flow rate

N, steady state response of parasympathetic nerve stimulation; s1, peak response to superimposed sympathetic nerve stimulation; s2, steady state response to superimposed sympathetic nerve stimulation; ph, phentolamine (0.1 mg kg⁻¹); yoh, yohimbine (0.025 mg kg⁻¹); pra, prazosin (0.025 mg kg⁻¹); pro, propranolol (0.1 mg kg⁻¹). * P < 0.05, when compared to the steady state response of parasympathetic nerve stimulation (N); + P < 0.05, when compared to the response with superimposed sympathetic nerve stimulation; n = 8.