Objectives. Functions of smooth muscle. Smooth muscle. Smooth Muscle Contraction: Mechanism. Latch state. Smooth muscle contraction

Transcription

1 Objectives Functions of smooth muscle Sompol Tapechum,, M.D., Ph.D. Department of Physiology Faculty of Medicine Siriraj hospital อธ บายล กษณะการหดต วของกล ามเน อเร ยบได อธ บายกลไกและป จจ ยท ม ผลต อการหดต วของกล ามเน อเร ยบได acetylcholine adrenaline extracellular Ca Smooth muscle Smooth muscle contraction Thin Filament Intermediate Filament Thick Filament Dense Mechanical Body Coupling Gap Junction Ca ++ (Contracts Smooth Muscle) + Calmodulin MLCK Inactive Ca Calmodulin camp (Relaxes Smooth Muscle) camp-pka Active MLCK-P Inactive and Ca ++ -Calmodulin Insensitive Longitudinal Section Cross Section Actin + Myosin (Relaxed) Ca Calmodulin-MLCK active ATP Actin + Myosin-LC P -ATP Head Detachment Myosin Light Chain Recock Head 90 Phosphatase o Cross ATP Bridge Cycling ADP + P i Power Stroke Actin-Myosin-LC P -ADP-P Latch state Smooth Muscle Contraction: Mechanism This is a special state that allows smooth muscle to maintain tone (force) with minimal expenditure of ATP. Stimulation + Ca Calmodulin-MLCK active Actin + Myosin ATP Actin + Myosin P -ATP (Relaxed) Myosin Light Chain Phosphatase Head Detachment Recock Head 90 o Cross ATP (low ATPase activity) Slow Detachment Bridge Cycling (high ATPase activity) ADP + P i (Latched) Power Stroke Actin-Myosin-ADP-P Actin-Myosin P -ADP-P Phosphatase 1

2 Smooth Muscle Relaxation: Mechanism Control of intracellular R neurotransmitter or hormone R voltage-operated ion channel ion exchangers and pumps K + ATP ATP 2nd messengers Na + Na + camp IP 3 cgmp regulation of kinases, phosphatases s.r. contraction phosphorylation of myosin Duration of smooth muscle contraction Excitation-contraction contraction coupling Depolarization Smooth muscle action potential is slow and carried primarily by through voltage-gated gated channels (L-type) Spike potential can be achieved if more channels open Repolarization is due to K + efflux through voltage-gated gated and ATP- sensitive K + channels Receptor-mediated [Ca 2+ ] i increase mechanism by which hormones, mediators, neurotransmitters and drugs act on smooth muscle. Smooth muscle of GI tract Single unit Membrane potential oscillation (slow wave potential) Tonic activity can be related to slow membrane potential changes Action potentials and twitches can be superimposed on rhythmical activity Tone = constant and stable low level of contraction Smooth muscle contraction 1. Slow wave potential Determines the smooth muscle tone Frequency of slow wave determines the frequency of contraction 2. Spike potential Number of spikes indicates the amplitude of contraction Smooth muscle electrical activity Smooth muscle contraction 2

3 Smooth muscle : Slow wave Smooth muscle lack the mechanism for active regeneration of slow wave. Slow wave decay in amplitude as predicted by passive cable properties in smooth muscle. Slow waves depolarize the smooth muscle cells, and activate voltage-dependent channels (L-type). - If threshold depolarization is achieved, action potentials are elicited. - If threshold is not achieved, the activation of inward current in smooth muscle cells is manifest as an increase in the plateau phase of slow waves. Depolarization also activate delay-rectifier K + channels which dampen the depolarization can causes repolarization. Pacemaker The slow wave potential in smooth muscle is initiated in Pacemaker cell called interstitial cell of Cajal (ICC). The ICC are electrically coupled and also electrically coupled with smooth muscle. Slow wave: Interstitial cell of Cajal ICC networks in pacemaker regions express the ionic mechanism to generate slow waves. No single ICC serves as a fixed, dominant pacemaker. Slow wave propagates at rate 5 mm/s Slow wave propagation in ICC network is regenerative Slow waves electrotonically conduct into smooth muscle cells, which are electrically coupled to the interstitial cells of Cajal (ICC). Mechanism of pacemaker activity Nonselective cationic channels (NSCC) -inhibit NSCC Mitochondrial Uptake Localized release of from IP3 receptors channel Activation of K Ca Low Reduce influx Repolarization NSCC inhibition Increase NSCC activation influx T-type L-type Cationic influx Depolarization Modulation of slow wave Expeimental setup Neural inputs can condition the electrical activity at both the interstitial cell of Cajal and the smooth muscle. - Release of excitatory transmitters activates nonselective cation channels and increases the effectiveness of slow waves to bring the muscle cells to threshold. - Release of inhibitory transmitters activates potassium channels, which decreases the probability of reaching threshold. PowerLab Force transducer Smooth muscle ring 3

4 Specimen Frog s s pyloric ring Experiment Characteristic of smooth muscle contracti tion Effects of -free solution Acetylcholine Adrenaline Cold K + Ba 2+ Control 5 minutes Experiment 5 minutes Wash twice Results Report Amplitude mv Frequency = contraction/minute amplitude Time tone Sec อ ณหภ ม สารละลาย 1 สารละลาย 2 สารละลาย 3 สารละลาย 4 สารละลาย 5 ยา Control 1 Experiment Control 2 Amplitude Frequency Amplitude Frequency Tone Amplitude Frequency Tone (mv) (/min) (mv) (/min) (mv) (/min) Effect of low extracellular Ca Discussion Effect on ICC The depolarization needs influx Low slow and less slow wave Effect on smooth muscle cell Less entry Effect on smooth muscle contraction Lower amplitude and tone Lower frequency? 4

5 Effect of acetylcholine Effect of ACh on ICC Activation of non selective cationic conductance (PKC) Increase frequency of slow wave Effect of ACh on smooth muscle cell GI smooth muscle expresses M2 and M3 muscarinic cholinergic receptors Increase intracellular (mediated by IP3) Suppression K + current (mediated by DAG) Reduce inactivation of MLCK Effect of ACh on smooth muscle contraction Increase frequency, amplitude and tone Effect of adrenaline Effect of adrenaline on ICC ICC do not express adrenergic receptor Adrenergic receptors are expressed at cholinergic terminal Adrenaline inhibits acetylcholine release Effect of adrenaline on smooth muscle cell Smooth muscle cells express β-adrenergic receptors (increase camp) Decrease intracellular (increase SR uptake) Membrane hyperpolarization (increase K + conductance) Inactivate MLCK Effect of adrenaline on smooth muscle contraction Decrease frequency, amplitude and tone Effect of adrenaline Effect of extracellular Ba Ba 2+ Effect of Ba 2+ on ICC and smooth muscle cell Ba 2+ blocks some K + channels (K( ir ) Membrane depolarization Increase intracellular Ba 2+ enters channels and acts as Ba 2+ on smooth muscle Increase amplitude, tone and frequency Effect of Ba smooth muscle contraction Effect of cold Effect of extracellular K + Effect of cold on ICC and smooth muscle cell Decrease cellular activity Decrease ATP production DecreaseNa + -K + pump activity Decrease pump activity Effect of cold on smooth muscle contraction Decrease contraction Initailly, tone may increase due to residual ATP and Effect of K + on ICC and smooths muscle cell Sustained membrane depolarization Effect of K + on smooth muscle contraction Initially, may increase contraction Later, decrease contraction 5

6 Conclusion Smooth muscle can contract without nerve supply. The contraction is initiated by intrinsic pacemaker. The contraction is slow (duration over 200 ms). Extracellular is important for the contraction. (Ca 2+ free solution) Neurotransmitters modulate smooth muscle contraction. 6